Dangermouse,
It doesn't disengage if you are AEO. The Nr just keeps on drooping.
The Tq limiter is designed for use when OEI training. If you screw up and droop the Nr too low, the OEI training switch kicks back automatically to the centre, and bang! you have full power very quickly. Without the torque limiter, you could put huge stresses into the gearbox.
I can't imagine why you would want Tq limiter other than for OEI training, but I am aware that some people do use it as a routine. Doh!