Full-Power Super Saiyan (超サイヤ人フルパワー) is the mastery of the Super Saiyan form, completely unrelated to the 2nd and 3rd Grade Super Saiyan stages of the transformation. The form is the Super Saiyan state trained into a "natural" form, and removes the agitation that is present in the original Super Saiyan form, also allowing the Saiyan to easier manipulate their ki in the form and utilize it more efficiently, minimizing strain. The idea is thought up by Goku, who insists he and Gohan use it, which they do to great success. The term "Full-Power Super Saiyan" was first created in the Daizenshuu guidebooks to differentiate it from the normal Super Saiyan form; in the actual series, it has never been referred to as such.
The Full-Power Super Saiyan form
"What's happened to them? They look like Super Saiyans, but I don't feel the energy... It's like it's their natural state..."
In the real world, athletes who train in aerobics for extensive periods of time experience heightened states of rest: heart and respiratory rate are much lower than that of a normal person. This Super Saiyan form follows a similar fundamental concept: by remaining transformed for extended periods of time, the Super Saiyan can desensitize themselves to the negative effects of the transformation. Their wild emotions are kept in check, resulting in a more strategic fighting style, and ki consumption is reduced to negligible amounts. Energy radiation is reduced in tandem, and the aura takes on a smoother, flow-like appearance. By mastering the transformation, the Super Saiyan is able to retain transformation almost subconsciously. In requiring less ki to contribute to maintaining the transformation itself, the maximum amount of ki can thus be focused into attacks and stamina use, with no adverse effects present in the 2nd and 3rd grades. Vegeta notes the incredible side effects of the transformation by stating the feeling Goku or Gohan's Full-Powered Super Saiyan form isn't the same feeling he gets around a normal Super Saiyan. A downside to this form is the difficulty that comes with controlling their strength in that form. While Goku and Gohan were drinking orange juice from glasses, they repeatedly broke the glasses from the difficulty of controlling their strength.
The key to this state is energy conservation rather than boosted energy output. This results in a Saiyan who is able to fight longer and more efficiently than with the 2nd and 3rd grades of Super Saiyan, and can even remain transformed for days without powering down as long as they do not expend too much energy, essentially making the transformation the Saiyan's base form. Following the creation of this concept and the introduction of Super Saiyan 2, the 2nd and 3rd grades are rendered obsolete. Because the Full-Power Super Saiyan form was a learned state of being, and not necessarily a typical transformation or power stage, it was used throughout the remainder of Dragon Ball Z by Goku and Gohan.
Whether there are intended to be physical differences in a Full-Power Super Saiyan from those of a standard Super Saiyan is debatable, because most of the physical differences can easily be explained as changes in Akira Toriyama's artwork as the series continued. The only confirmed difference is that the Saiyan's facial features exhibit less tension and ferocity than the regular Super Saiyan form, which can be explained by the mastery over the ki consumption and the emotional restlessness that the regular Super Saiyan transformation causes. The aura is also somewhat different: whether or not it was by accident, the aura is seen to be less violent than the second and third grades of the Super Saiyan. When relaxed, the user's hair appears more of a yellowish white, but when powered up, the golden glow seen in the original transformation returns.
Usage and power
Goku and Gohan were the first to achieve the Full-Power Super Saiyan state while spending time training in the Hyperbolic Time Chamber, in which Goku realized they must "... work on getting rid of that instability... that restless feeling.", in a sense, regular Super Saiyan must feel like their normal forms. They spend the remainder of their time within the chamber and ten days before the Cell Games maintaining their Super Saiyan forms until they felt relaxed, a mastery of the Super Saiyan form. After having achieved this mastery, Goku and Gohan were able to freely modify their power levels as they would in their base forms. Their bodies' familiarity with the transformation enabled them to fight with greater stamina while at the peak of the normal Super Saiyan limits. When Piccolo, Tien Shinhan, Future Trunks, and Vegeta are discussing Goku and Gohan's states as Super Saiyans, Future Trunks assumes that Goku and Gohan will undergo another huge transformation during battle, but Vegeta rebukes him, saying "They decided that this is their best combat state! If they get used to it, they'll minimize the strain on their bodies during battle!" For the duration of the series, Full-Power Super Saiyan is not referred to as such, only as "Super Saiyan."
The form was first seen during the Cell Games, Goku battled Perfect Cell in what seemed to be an even fight, with no apparent advantage on either side. Goku eventually forfeited the match and had Gohan step in to take his place. Perfect Cell initially had an advantage over the young Super Saiyan, mostly because of Gohan's gentle nature and thus his refusal to go all-out, and began to goad Gohan into utilizing his full strength (something Goku was counting on when he ended his own fight). After Perfect Cell unleashed seven clones of himself to attack the Z Fighters, Gohan withstood less and less of the torture and his anger began to rise. It was not until Perfect Cell killed the peace-loving Android 16 that Gohan lost all self-control and transformed into a Super Saiyan 2, allowing him to defeat the Cell Jrs. and later Cell himself.
The Full-Power Super Saiyan form would be used by Goku and Gohan for the remainder of the series as their normal form for transforming. However, they would usually have to transform into a higher state in order to combat the more powerful foes, such as Majin Buu. There are hints that Vegeta and Future Trunks have also achieved this state as early as their second visit to the Hyperbolic Time Chamber before the Cell Games (in the manga, they are seen in their regular Super Saiyan forms against the Cell Juniors, while in the anime they possibly use the 2nd Grade Super Saiyan form instead). Later, in his own timeline, Future Trunks is seen with the same aura as Goku and Gohan had, along with a slightly changed hairstyle to further evidence this. Goten and Trunks also seem to be very relaxed and calm when turning Super Saiyan.
In the film Dragon Ball Z: Broly - The Legendary Super Saiyan, Goku and Gohan battled Broly in this state, only to be badly beaten by the Legendary Super Saiyan. In the film Dragon Ball Z: Bojack Unbound, Gohan again fought using the Full-Power Super Saiyan form and was able to keep up with the Galaxy Soldiers. However, he had to transform into a Super Saiyan 2 afterward to defeat Bojack and the Galaxy Soldiers.