Helicopter torque

There's two options:

Firstly, the tail rotor. The primary use for this is yaw control, including correcting for torque from the main rotor, so if this is still running it will over-correct and cause the rotation you're talking about.

Secondly, friction. In a situation where the helicopter is autorotating (behaving like an autogyro), if the gearbox is still engaged there is a lot of mechanical drag which will cause torque. The energy source for this is airflow, so the aircraft needs horizontal and/or vertical airspeed.