Other source:
The match between Trunks and Goten begins to heat up. Trunks gets Goten in a stranglehold, and Goten can't seem to break free. Desperate, he transforms into a Super Saiyan and busts loose. Trunks is angry, because all of the Saiyan warriors agreed not to transform during the Tournament. To embarrass him for this, Trunks decides to fight the rest of the way without using his left arm. But when Goten seems to have the advantage, he uses his left arm AND transforms into a Super Saiyan to knock Goten out of the ring. Trunks says that since both he and Goten transformed once, they're even.
