An external device, e.g., an electric motor, supplies constant power to a rotating system, e.g., a flywheel, through a torque $\mathrm{\tau }$ . The angular velocity of the system is $\mathrm{\omega }$. Both $\mathrm{\tau }$ and $\mathrm{\omega }$ are variable

The force that can cause an object to revolve around an axis is known as torque. In linear kinematics, force is what drives an object's acceleration. Similar to how that happens, torque accelerates an angle. Thus, torque can be thought of as the linear force's rotational counterpart. 

We know that power,

$\mathrm{P}=\mathrm{\tau \omega }$

Since power is constant

$\mathrm{\omega }\propto \frac{1}{\mathrm{\tau }}$