I have never flown with a stalled horizontal stabilizer, and it sounds to me like a maybe non-recoverable situation with uncontrolable increasing pitch down. But here is my guess.
For a plane flying straight and level the lift on the wings is up while the lift on the horizontal stabilizer is down. This is required for pitch stability, and is a consequence of the center of lift on the wings moving forward as the angle of attack increases. Without the downward force on the tail the aircraft would be unstable in pitch (unless it had a canard).
So if the tail stalls due to icing the downward "lift" needs to be increased somehow, and I guess up elevator might be one way of doing this. Pulling on yoke should
decease the AoA on the horizontal stabilizer
Maybe someone who really knows the suject can expand on this.
Cheers,
