Originally posted by Jedi Master:
My guess is one rotor increases blade AOA while the other lowers it. This keeps lift constant while inducing a moment. I don't know for certain how it works on tandem rotors like the CH-46 and CH-47, but there it's probably one rotor tilts left and the other tilts right.
The Jedi Master
You're right about that for coaxials. One will increase collective, the other decrease. This causes one disk to create more torque, the other less, and the chopper yaws.
For inline (CH-47) or side by side (Ka-22, Mi-12, MV-22 when pretending to be a helo) tandems simply cyclic the rotors opposite to each other. In a Chinook, if it wanted to yaw left, the forward rotor would get a 'roll' input to the left, and the aft rotor a 'roll' input to the right.