Law of Conservation of Angular Momentum

Newton’s second law for rotational motion

So if the net external torque on a particle (or system) is zero then

i.e.                       = constant.

Angular momentum of a system (may be particle or body) remains constant if resultant torque acting on it zero.

  As L = Iω so if  Iω = constant            ∴ I ∝ (1/ω)

Since angular momentum Iω  remains constant so when Idecreases, angular velocity ω increases and vice-versa.