Dragon Ball Kai EP82 - The Strongest Super Saiyan! Trunks' Power Unleashed!

Trunks powers up to Ultra Super Saiyan, which appears to make him significantly stronger than Vegeta. Trunks and Cell seem to be evenly matched at first, but after a while Trunks' attack cease to hit Cell. Trunks decides to power up even more but he is still unable to connect his attacks to hit Cell. In the Hyperbolic Time Chamber, Goku manages to power up to the same bulked-up transformation as Trunks, but decides not to use that form, because in order to achieve that form, even though great power is gained, Goku concludes the decrease in speed is too much - and having power but not being able to hit your opponent is useless, so he decides to focus on mastering his normal Super Saiyan form instead. This turns out to be the same reason why Trunks cannot defeat Cell, and Trunks power level begins to drop due to the strain of maintaining his transformation. Cell learns that Goku is training to defeat him, so in order to waste time he decides to hold his own World Martial Arts Tournament where anyone can enter to fight him.