With your arms closer to your body you can spin faster than with arms spread. This happens because of the conservation of angular momentum. You and the platform form a system. Angular momentum is a quantity related to the system’s mass, its size (those quantities make up the moment of inertia) and the rotational speed. Mass of the system remains the same, so when its size increases, rotational speed decreases and vice versa – smaller size means higher rotational speed (called angular velocity).