Law of Conservation of Angular Momentum :
(1) If the net external torque on a particle (or system) is zero then
Angular momentum of a system (may be particle or body) remains constant
if resultant torque acting on it is equal to zero.
As L = Iω, so if , then Iω = constant ∴
Since angular momentumIω remains constant so when I decreases, angular velocity ω increases and vice-versa.