What Limits
I think the rotorbrake mod would not close the throttle. If the brake was applied at flight idle. The mod would prevent operation of the brake.
The throttle is gated in the fully open open position and the only way to close it would be to haul on the brake with enough force to break it out of the gate.
I think the mod is to prevent operation of the brake at flight idle and to prevent throttle advance with the brake on.
However it's all a long time ago so my memory could be at fault.