in chapter 3, what did gene do to his own disbelief and why did he do it?

The first rule is that Finny and Gene must jump from the tree at the beginning of every meeting; Gene cannot get used to this daredevil stunt, though he has done it many times before. The club meets every night, because Finny deems it so; Gene doesn't want to go every night, or do everything Finny wants to, but follows Finny anyway because of their friendship.