Conservation of angular momentum:

You can see this for example if you watch gymnasts, divers off high board or jumpers on a trampoline doing a somersault.

They gain momentum and then put their arms and legs very close to their axis of rotation. The angular momentum is conserved because there is no impact of an external force. They decrease their moment of inertia by displacing mass inwardly close to the axis of rotation. Thus their angular velocity must increase in case of constant angular momentum. This means that they are turning faster.

You can see this by watching that person doing a somersault: