Poor Bruce. He's the slightly older boy who steals some of the Trunchbull's cake and is punished by her in front of an entire assembly. Except, it doesn't turn out too bad for the kid, and in the end, Bruce comes out on top. He stands up to the Trunchbull, and makes her punishment lose effect.
Here's how it all goes down: the Trunchbull tries to force Bruce to eat a cake that's eighteen inches in diameter (as a punishment for swiping some of her cake earlier). It's bigger than his head, and no reasonable person would be able to eat more than a quarter of it. Probably. The Trunchbull is hoping that he'll eat until he's ill; he'll have too much of what he wanted, and not on his own terms.
But her plan fails in spectacular fashion. Bruce rises to the challenge, showing he has hidden depths—at least, as a competitive eater. Once he gets going, he keeps going. And going. And going.
When Bruce stuffs the last piece of that giant cake in his mouth, he's not just doing it for himself. He's showing all the students there that they can fight against the Trunchbull's crazy punishments, too. It's a total triumph, and the entire auditorium bursts into applause. In that moment, Bruce turns the Trunchbull's punishment upside down and undermines her authority. Awesome.
It's possible that seeing Bruce do this gives Matilda courage. He becomes a role model when it comes to standing up to adults and fighting for the rights of kids everywhere. It's as if he looked the Trunchbull right in the eye and said, bring it on. Do your worst, you old meanie. This seems to be the moment that Matilda embraces this idea of fighting back with hope of success. After all, if Bruce (a silly, over-eating boy) can stand up to the Trunchbull in such a big way, why can't she?
After the Trunchbull smashes the cake platter over his head, that's about the last we see of Bruce. We can remember him fondly, especially as portrayed in Quentin Blake's illustration, sitting on the stage in the assembly hall, absolutely replete and stuffed to an inch of his life with chocolate cake. (For more about Bruce and what his eating of the cake represents, head over to the section "Symbols, Imagery, Allegory.")