How was the super suicide formed for the summer session?

This is just an informal "club" of boys who like to do dangerous stunts. To start off their meeting of the Super Summer Suicide Society, they decide to do jump off the tree together. Once they are on the tree, Gene "jostles" the limb, and Finny loses his balance and falls; Gene, unshaken, jumps into the river, seemingly without remorse or concern for his friend.