Other source:
As Vegeta battles Cell, it is clear that Vegeta has the upper hand in the fight, his strength superior to Cell. Watching from the sidelines, Trunks recalls the intense time he spent training with his father, in the Hyperbolic Time Chamber. As the fight between Cell and Vegeta continues, the pressure falls on Gohan as he trains in the time chamber with Goku in an attempt to become a Super Saiyan.
