In The Chosen, what painful thing does Reuven do just before the game starts?
The answer to this question can be found in the first chapter, just after the start of the novel. Reuven explains how, before playing every game, he has to bend the earpieces in of his shell-rimmed glasses so that they would stay tight on his head and not slip down his nose when he began to sweat. Even though this is a painful thing for him to do, and he suffers afterwards, it allows him to not be distracted by his glasses constantly slipping and helps him to maintain focus on the match. Note what Reuven says about it:
I always waited until just before a game to bend down the earpieces, because, bent, they would cut into the skin over my ears, and I did not want to feel the pain a moment longer than I had to. The tops of my ears would be sore for days after every game, but better that, I thought, than theneed to keep pushing my glasses up the bridge of my nose or the possibility of having them fall off suddenly during an important play.
Reuven then deliberately bends the earpieces of his glasses in, in spite of the pain that this gives him, so that he can focus on the match and not get distracted.