The Strongest Super Saiyan! Trunks' Power Unleashed! (「超パワー覚醒！父を超えたトランクス」, Chō Pawā Kakusei! Chichi o Koeta Torankusu; lit. "Super Power Awakening! Trunks Surpasses his Father") is the eighty-second episode of Dragon Ball Z Kai. Its Japanese air date was November 21, 2010. Its American air date was October 20, 2011.
Future Trunks and Cell seem to be evenly matched at first, but after a while Trunks' attacks cease to hit Cell. So, 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 level of power as Trunks, but decides not to use that form because in order to achieve that form, even though great power is gained, Goku concluded the decrease in speed is too much (due to the large muscles) and having power but not being able to hit your opponent is useless so he decides to opt for his normal Super Saiyan form.
This turns out to be the same reason why Trunks cannot defeat Cell. Cell learns that Goku is training to defeat him so in order to waste time he decides to hold a martial arts tournament where anyone can enter to fight him, the Cell Games.