What was one of Finny's favorite tricks?

From the text:

As I had to do whenever I glimpsed this river, I thought of Phineas. Not of the tree and pain, but of one of his favorite tricks, Phineas in exaltation, balancing on one foot on the prow of a canoe like a river god, his raised arms invoking the air to support him, face transfigured, body a complex set of balances and compensations, each muscle aligned in perfection with all the others to maintain this supreme fantasy of achievement, his skin glowing from immersions, his whole body hanging between river and sky as though he had transcended gravity and might by gently pushing upward with his foot glide a little way higher and remain suspended in space, encompassing all the glory of the summer and offering it to the sky.