The answer...He shifts his weight using his hips. Remember,riding a bike is a balancing act.You must cause the bike to fall one way or the other. You can do this one of two ways...one way is to turn the handle bars in the opposite direction. Turn them left (very slightly) and the steering trail built into the geometry of the front fork will cause the bike to fall to the right and vice versa. The other method is to shift weight from side to side. The upper body will stay centered and the weight shift occurs in shifting the hips from one side to the other causing an imbalance and the bike will begin to fall.
A simple experiment...balance a broom with the heavey end up in the air and the pole placed in the palm of your hand.
(the more weight up at the top end the easier it will be to balance)
Take notice that (while your balancing) if you want to move the broom to the right you must first move your hand left to create the imbalance that starts the broom falling to the right.
It can't be done any other way.
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