In the absence of any external torque being applied angular momentun is conserved. A spinning object's angular momentum is equal to its moment of inertia "I" times its spin rate in radians/second, w. Moment of inertia is a function of the distribution of the object's mass as measured from the axis of rotation - the further mass is from the axis of rotation the greater its value of I. If a spinning system "contracts" its moment of inertia decreases, as mass is moved closer to the axis of rotation. Since I decreases, and the angular momentum stays constant, the speed of angular rotation w must increase. The classic example of this effect is the spinning ice skater who starts with her arms spread, and then as she pulls her arms in her rotational speed increases.