Other source:
Goku and Yakon start to fight. Goku is obviously stronger than Yakon, so Babidi changes the environment to mimic the Planet of Darkness, which is Yakon's home planet. To help himself see, Goku becomes a Super Saiyan. Yakon, the devourer of light, sucks up the light emitting from Goku. Seeing this, Goku again becomes a Super Saiyan, allowing Yakon to suck up all the light energy he can. Finally, he gives off a flash of brilliant energy which is too much for Yakon to handle. Victorious, the fighters head to the next level.
