[imglorp]

I think it's about loosing a rotor, not net torque. You need to keep the roll and pitch stable in an outage.

Like that hexapod in the above link, if you lose one prop/motor/controller, you can increase power to other unit in the stack and decrease power to all the others, maintaining your stability while you descend and look for an emergency landing spot.

[baddox]

Right, but if both of your rotors are spinning the same direction, you have net torque under normal operation.

[imglorp]

Only from the one pod. In all these multirotor deals, there will be other pods to make a net zero torque: not necessarily all the same pod to cancel itself out.