Saiyan Power, known to the fandom as Zenkai (全開 Zenkai, lit. "Full Release" or "Full Throttle"), is a Saiyan genetic trait that allows their performance to continually increase against adversity. This is done by the Saiyan imprinting on struggles and instantly learn from it. As it does, their minds and body adapt to the said struggles, allowing their instinctual methods and natural resistance to noticeably improve, which also increasing their physical might as they recover, thereby lessening the chances of the problem happening again. The more severe the damage they recover from, the more significant the improvements will be.
"Around and around he goes...working his body and chi almost to death and then refreshing himself by eating the Senzu... Whether he realizes it or not, Goku is living and reliving one of the primal cycles of the Saiyans... Gaining strength by facing and overcoming death!"
— Narrator - Premonitions of War
After drinking the Ultra Divine Water and almost dying, Goku presumably got a power boost from not just the water's powers, but also from his Saiyan Power, as seen from the silhouette of his Great Ape form. During the Namek and Frieza Sagas, Saiyan Power is an extremely important element, which allows Goku, Gohan, and Vegeta's powers to increase in order to "catch up" to Frieza and his men. In the Cell Games Saga, when Cell returned from his botched self-destruction on King Kai's planet, his power amplified to the point that high amounts of energy surrounded his body to mimic the aura of a Super Saiyan 2.
Saiyan Power seems to increase exponentially after each use. When Vegeta recovers from his first fight with Goku, his power level went from 18,000 to 24,000, and another was received after healing from being nearly killed by Zarbon, reaching power level of 30,000. After Vegeta was severely injured by fighting Recoome, his strength skyrockets to match First Form Frieza (power level 530,000) in a power struggle. Another usage allowed Vegeta to see Fourth Form Frieza's movements and Death Beams at first, before he begins to put in even more effort. Goku's usage of Saiyan Powers during his gravity training increased his power level from 8,000 to 90,000, and again after recovering from his battle with Captain Ginyu (power level 3,000,000).
Goku experiences another boost after recovering from Cooler's Darkness Eye Beam, able to just overwhelm the foe who previously grievously injured him with a single attack in the movie Dragon Ball Z: Cooler's Revenge. In Broly - The Legendary Super Saiyan, it is stated that Broly survived his execution and grew stronger due to his Saiyan genes. In Broly - Second Coming, it was noted by Gohan that Broly grew significantly stronger after recovering from his lost against Goku. The fact that Broly appeared stronger in only Super Saiyan than his previous usage of Legendary Super Saiyan implies that Broly tapped into Saiyan Power.
In Bardock - Father of Goku, it was noted by doctors that which each mission he returned from, especial when needing medical attention, he was showing considerable increase in power. It was even noted prior to the Genocide of the Saiyans that Bardock's power has come to rival King Vegeta. In The History of Trunks, originally Super Saiyan Future Gohan struggled against even 1 of the evil androids (Future Android 17 even noted that he used less than half of his power). By the battle at the Super World amusement park, Future Gohan was able to fight on par with Future 17 and gradually overwhelm him, shocking the foe and forcing him to team up with Future Android 18 to defeat him. Later after recovering, despite having lost his left arm, Future Gohan proved able to overpower both androids simultaneously at their full power, ultimately forcing the tireless androids to instead exhaust Future Gohan to defeat him. These improvements for Future Gohan seemingly came from accessing Saiyan Power.
In Dragon Ball GT, During Goku's battle against Eis Shenron he was frozen solid by the Shadow Dragon. Originally, Goku required Nuova Shenron's aid to escape. Afterwards however, Goku became completely unaffected by the foes attacks next usage as he instantly broke free while completely unfazed from the event. While Eis was left stupefied, Goku noting firmly that the same technique does not work twice on him.
In Dragon Ball Super, Goku Black usage of Saiyan Power is shown to be noticeably more potent than normal Saiyans. This was possibly due to his natural godly ki. From this, each battle took him to much higher levels and ultimately allowed him to obtain his Super Saiyan Rosé and likewise enhance his techniques. While in the manga version Black's required Future Zamasu's aid to recover fast enough to boost his might, in the anime, Black Saiyan Power also allowed him to recover in short periods of time and gain considerable boosts from even mere pain. This was seen from being being heavily damaged by Goku in only a Super Saiyan 2 state, but upon recovering, he was able to fight on par with Super Saiyan Blue Vegeta in his base state while virtually unfazed by the latter's attacks.
During the Tournament of Power, originally Super Saiyan Blue Goku was unable to repel the assault from Berserker Super Saiyan Kale. As he began recovering his stamina from his fight against Jiren, Goku was able to calmly overpower True Legendary Super Saiyan Kale and Super Saiyan 2 Caulifla simultaneously. Similarly, Caulifla, who was a crude brawler with little tactical skill, began quickly adjusting to Goku's more experienced maneuvers, learning more efficiency in her combat skills methods as Goku continued to pressure her, soon forcing Goku to fight her on the same level of Super Saiyan 2.
During the Frieza Saga, Vegeta tells Krillin that a Saiyan cannot increase their power if the wound is self-inflicted, which is why Vegeta made Krillin injure him so Dende could heal him and grow stronger. However this limitation is disputed by Goku's power increase power while healing from his extreme gravity training and by Cell powering up after his self-destruction. One possible explanation for Goku's boost during his training is it came from a blast he didn't control. Another possibility is that the extreme gravity training took its toll on Goku. In the case of Cell's self-destruction, it is possible that his Saiyan attributes were improved akin to his Namekian attributes allowing him to regenerate well-beyond a true Namekian.
There also appears to be a lack of consistency on how potent the increase in power will be. After the Frieza Saga, any power-ups even from near-death by the Saiyan Z Fighters became so minor that they were unnoticeable. In the Fusion Saga, it is noted by Super Buu that after Gohan is healed by Dende, he showed no improvement against the enhanced Majin. In the Dragon Ball Super manga, Future Trunks theorizes that Saiyans can only increase their power from this ability up to a certain point. Future Trunks came to this conclusion due to Goku and Vegeta having already trained themselves well-beyond normal Saiyan limits as the latter showed no signs of getting stronger after healing from his beating against Goku Black.
Video game appearances
In the RPG Dragon Ball Z: Super Saiya Densetsu, when a Saiyan reaches less than 5 HP, using a full heal item on him (like a card of Kami) will result in him receiving a Saiyan Power and raise his level by 1. This can be used early on against Raditz, to boost Goku's level extremely high. Also in the game, there is a glitch with Saiyan Power: if a player chooses to level Dende to level 3 (9999 exp earned twice), Dende can heal a character as well. Should he heal a Saiyan that is at 5 HP or below, the Saiyan receives the Saiyan Power, but not the level up. This allows certain characters like Gohan or Vegeta to be boosted to higher-than-usual power levels. Using the Moon Card on Gohan, however, restores his power level to the level it should be.
In Dragon Ball Z: Battle of Z, a customize card named "Saiyan Power" can be equipped onto a character and raises their attack power every time they are knocked out. This card is based on Saiyan Power.
In Dragon Ball Xenoverse, Saiyan Power is one of the Saiyan race's unique traits. When a Saiyan falls in battle, and is revived, either through teammate assistance or a Battle Item, they gain a large power boost that lasts for the duration of that battle. A Saiyan Power effect can be applied to non-Saiyans through Z-Souls such as Vegeta's "Saiyans are a Warrior race!" or through Super Perfect Cell's "That was a handy miscalculation".
"How many life-or-death battles have you been through? After all, Saiyans get stronger when they're on the verge of death. You keep getting stronger, too, so you must've nearly kicked the bucket any number of times, right? I've also nearly died... or should I say, actually died a number of times, heh heh heh."
— Goku talking about Saiyan Power with the Saiyan Future Warrior in Dragon Ball Xenoverse 2
In Dragon Ball Xenoverse 2, Saiyan characters can receive Saiyan Power in the same manner as they did in Xenoverse.
During the Unknown History Saga, as Super Saiyan Xeno Bardock fights with Mira inside the time rift he had trapped himself and Mira within, Mira notices that Bardock is growing stronger as he fights and combined with the alterations Towa had done to his body while he was brainwashed as the Masked Saiyan, it is implied that it allows him to ascend first into Super Saiyan 2 and before finally transforming into his Super Saiyan 3 form, allowing him to defeat Mira before his mysteriously disappears from the time rift, leaving the defeated Mira behind but alive.
As part of the Super Pack 1 DLC, Cabba grows stronger like other Saiyan characters do in Xenoverse 2. Interestingly, he will reference this after being revived by saying "I feel great!". This indicates the Saiyans of Universe 6 also possess the ability to receive a Zenkai like their Universe 7 counterparts.
- The term Zenkai is used in the first Dragon Ball Z ending theme, "Detekoi Tobikiri Zenkai Power!"
- The Meta-Coolers' Auto-Repair Ability also corrects any flaw that allowed the unit to take damage, similar to Zenkai.
- Fused Zamasu's immortality in the manga is very similar to this, as when Fused Zamasu's time fused was up, Fused Zamasu's cells were breaking apart into Goku Black and Future Zamasu inside his body, akin to severe cellular damage. When Future Trunks sliced the half defused Fused Zamasu in half, Goku Black and Future Zamasu individually transformed into Fused Zamasu, unable to separate due to Goku Black and Future Zamasu both being the same being (Zamasu) from different timelines. And when Vegeta tried to split the two Fused Zamases in pieces, they instead formed into clones, becoming Infinite Zamasu. Fused Zamasu and Infinite Zamasu are both half-Saiyans.
- Hit's Pure Progress ability allowing him to grow stronger as fights is also similar to Saiyan Power. While only increasing his efficiency and versatility but not his raw might, his growth rate is seemingly faster than a Saiyans, as Vegeta reluctantly noted.
- Ganos' own usage of Pure Progress shows the opposite result. While not improving his skill or experience, his sheer might continued to noticeably grow in only minutes at a time.
- While the dub incorrectly implies that Saiyan Powers only occur with Elite Saiyans, Goku a Low-Level Saiyan, receives them many times. This is confirmed by how Krillin states how Goku always grew ever stronger after recovering from fighting devastating battles.