Saiyan Power, known to the fandom as Zenkai (全開; lit. "Full Release" or "Full Throttle"), is a Saiyan genetic trait that allows their combat prowess to continually increase as they fight. This is done by the Saiyan instinctively imprinting on new experiences to lessen the chances of same injury happening again and likewise dramatically increasing their raw might after recovering from injuries.
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 a Zenkai. He also possibly got one after his near death encounter with King Piccolo. During the Namek and Frieza Sagas, Zenkai is an extremely important element of the series, which allows Goku, Gohan, and Vegeta's powers to increase in order to "catch up" to Frieza and his men. After the Frieza Saga, most of the Saiyans had already become Super Saiyans. In the Cell 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.
Zenkai's 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 being nearly killed by Zarbon's transformed state and healed in a Medical Machine. After Vegeta was near-fatally injured after fighting Recoome, his strength strangely skyrockets enough to have an even power struggle with Frieza's first form (stated to be 530,000), and another, final Zenkai increased his power enough to see Frieza's movements and Death Beams at first, before he begins to put in even more effort. Goku was able to tap into the power of Zenkai's during his gravity training (which in total boosted his power from "over 8,000" to 90,000), and again receives an enormous boost after recovering in a Medical Machine from his battle with Captain Ginyu (increased to 3,000,000). To force a boost in power during the battle with Frieza, Vegeta tells Krillin to brutally injure him and have Dende heal him, which after greatly increased his power, though still not nearly enough to defeat the space tyrant (who by that point had just transformed into his most powerful state).
Goku experiences a Zenkai upon eating a Senzu Bean in the movie Dragon Ball Z: Cooler's Revenge. In Dragon Ball Z: Broly - The Legendary Super Saiyan, it is stated that Broly survived his execution and grew stronger due to his Saiyan genes. In Dragon Ball Z: Broly - Second Coming, it is implied that Broly grew significantly stronger while in a coma for seven years after being beaten by Goku and recovering.
In Dragon Ball Super, Goku Black is able to upgrade his power through this ability following each battle against Goku and the other Saiyans, thereby obtaining his Super Saiyan Rosé and likewise enhance his techniques.
Saiyans are not capable of infinitely gaining zenkais however, and the more zenkais the Saiyan gets the lesser the effect, each time to the point that they eventually do not get a power up from being injured anymore. After the events of the Frieza Saga, Goku, Vegeta and Gohan had had enough of these power ups that they no longer granted any noticeable boost. During the Fusion Saga, after Gohan is healed by Dende after being heavily injured by Super Buu, Buu notes that he has not powered up at all. In the Dragon Ball Super manga Trunks stated that the likes of Goku and Vegeta reached their current level just because of hard training as they no longer can use zenkais to power up.
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 Zenkai 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 Zenkai: 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 Zenkai, 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 Zenkai.
In Dragon Ball: Xenoverse, Zenkai 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 Zenkai 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 Zenkai with the Saiyan Future Warrior in Dragon Ball Xenoverse 2
In Dragon Ball Xenoverse 2, Saiyan characters can receive a Zenkai 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.
- Hit's Pure Progress ability allowing him to grow stronger as fights is also similar to Zenkai.
- While the dub incorrectly implies that Zenkais 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.
- In chapter 303, Vegeta states that a Saiyan cannot experience Zenkai if the wound is self-inflicted. However, Goku, in the anime received a Zenkai after healing from injuring himself with a Bending Kamehameha and Cell likewise did after regenerating himself from self-destruction. However, Goku's Zenkai may have come from his extreme gravity training and Cell's from his augmented/mixed Saiyan, Namekian, and possibly Frieza's race cells.
- It should be noted that this occurs before Vegeta reveals that it cannot be obtained by self-inflicted wounds.