Other source:
Goku instructs Krillin and Gohan to leave the battlefield, because he doesn't want them to get hurt. With no one in their way, Goku and Vegeta begin their fight for the fate of the Earth. Even with Goku using his Kaio-ken technique, Vegeta is still more powerful. Goku has no choice but to try a Kaio-ken x2, but even with this increase in power, he still can't overpower Vegeta. His only choice is to try a Kaio-ken x3, but the incredible surge in power could destroy his body.
