Hey Phil&Bill,
To the best of my knowledge they disengage the pre-rotator before the wheels leave the ground.
As long as the wheels are in contact with the ground anti-torque isn't an issue.
For non jump-takeoff pre-rotators the amount of torque is fairly small, Bensen
writes of having someone takeoff with the pre-rotator engaged and only noticing that
they were having to hold some rudder trim.
I think Bensen went on to design a model with a partially powered rotor
that just needed rudder to counter the torque.
mickey