Time and again beginning with the later dragonball episodes and lkater dbz, dbzsuper,...) we see ki warriors power up before they unleash their true powers / strongest attacks.
Now I'm wondering though...as it seems powerup was only introduced later on when ki sensing and ki suppression was introduced: Do ALL ki warriors need to power up to unleash their strongest attacks / full power? Or is it only necesary when you suppress your power level to power up first?
The answer to this question would be a Yes in terms of Dragon Ball Z and a Yes & No in terms of Dragon Ball Super.
Ki (気 "Kee", lit. "Spirit"), also known as chi/qi, Yōki (妖気)[1] or simply energy (エネルギー enerugī), is the life force energy used by Dragon Ball characters. Source
So as per the definition, it would make sense that the more Ki/ energy is released, the more powerful a fighter is or the more powerful their attack is. For example, if there is a beam struggle between two fighters, the fighter who raises his energy/ki more comes out on top. The greater the amount of energy released by a warrior, the greater is the level of power. So yes! Realistically, any character would have to release their ki to maximum to release a powerful attack.
The reason I said Yes & No in terms of Dragon Ball Super is because we see a bit of contradiction to the traditional Dragon Ball Z on account of the introduction of God Ki. God Ki involves high-level ki control where the user is not allowed to have Ki leak out of their body so that the opponent cannot sense their level of strength. In the Super Saiyan God and Super Saiyan Blue form, it isn't possible to directly sense the ki of the fighter. However, an experienced fighter is capable of gauging the opponent's strength and the form tends to exert immense pressure on whoever is fighting against it. The reason I say Yes and No is because, while this notion of God Ki is present, the old traditional concept I explained earlier still exists even in the case of the God forms. For instance, when Goku has a beam struggle with Merged Zamasu, he powers up more to overpower his attack, the same applies to when Goku is angry with regard to Chi Chi's and Goten's death. All of this is in their Super Saiyan Blue forms. Similarly, in the tournament of power, we see Goku power up completely and releases all his ki in Episode 122 before he faces off against Jiren. Vegeta does the same when he powers up his Final Flash against Jiren.
So realistically yes, the more ki a fighter has the stronger they are. Naturally, naturally, not all fights require fighters to release all their energy at one go to take on every foe. So yes, based on the opponent and his level of strength, the opposing fighter either powers up or down based by releasing ki.
