the tail motor only operates in one direction to counteract (or overcome) the turning tendency which is a byproduct of the lift generated by the main rotor blades. The force caused by the main rotor blades only tries to move the nose of the heli in one direction so the tail rotor only needs to be able to spin one direction to counteract this. By varying speed on tail rotor blades, less or more lift can be created by them to keep the nose straight. If the heli is trying to spin one direction or the other or you continously need your left stick to have an input to keep the nose from spinning use the trim and adjust it out a little. The tail rotor should stop when you move the stick all the way to the left to answer though. By doing so, it allows the gyroscopic force of the main rotor to turn the heli and if you want the nose to go to the right it will speed up.