Other source:
Cell is through playing games, so he decides to take out Gohan, and the Earth, once and for all. While powering up his Kamehameha wave, he explains how he became Dr. Gero's "ultimate perfection." Gohan thinks this is the end of the world, but suddenly Goku begins to speak to him telepathically through King Kai. He tells Gohan that he has the strength and that he can't give up. So, despite only being able to use one arm, Gohan powers up the biggest Kamehameha wave he can muster and aims it directly at Cell, just as Cell releases his wave.
