Like skaters spinning in place on the ice, they both take advantage of rotational dynamics. They start with their legs and arms out wide. During this phase they have a high momentum of inertia, meaning that each contestant's mass is distributed widely about his axis of rotation. When they bring their arms and legs in tight, transforming themselves into spinning tops, their moment of inertia decreases. This causes them to accelerate, cramming in more complete spins per second, because their angular momentum, which is a function of angular rotation and moment of inertia, wants to remain constant.