Why in helicopter autorotation phase the opposing torque is eliminated? As you many know, according to Newton's 3rd law for every action there is an equal and opposite reaction, as the main rotor of a helicopter turns in one direction, the fuselage tends to rotate in opposite direction but when there is no engine power during autorotation, there is no torque reaction, why???
Because the engine is not applying power to the rotors. No engine power means there will be no torque pushing the airframe in the opposite direction.

In auto, the main rotor essentially becomes a pinwheel, kept moving by the air rushing past it... 

Auto doesn't really power the rotors, it keeps them moving so that the spinning rotor creates air resistance and slows the descent of the helicopter, and maintains enough inertia in the blades to flare and stop the descent. 

Of course, that inertia is only sufficient for one attempt at flaring and stopping descent.