In simple words, torque is the cutting force which means that it depends on torque whether it will cut or route the toughest materials or not. So, more torque is needed for tough wood and vice-versa. RPM ( Rotation Per Minute ) is used to limit the router bit diameter. In a mathematical way, RPM ∝ 1/Diameter of the router bit. That means, always purchase a router with lower RPM if you are using a large diameter bit.