Helmet Fire,
The reason why the torque rises when the rpm is bled off is actually quite simple:
It is not because the velocity is lower (which it is) because the collective rises to make more alpha for the lower air velocity, and the lift is a constant (as measured by the fact that the aircraft neither accelerates upward nor downward). In actuality, the rotor is almost exactly as efficient at lower rpm as at higher (same lift for same power) in fact, the rotor on most helos gets a tad better at lift for power as the rpm is reduced a small amount.
The simple fact is that the rotor is eating power, not torque, and at lower rpm, more torque is needed to generate the same power.
Torque is how much twisting force the shaft needs, and is measured in foot-pounds (a unit of torque is the twist exerted by a force of 1 pound exerted one foot from the center of the shaft (like a torque wrench). If the shaft is not turning, you can still exert a torque on it, but it takes no power to do so. If you exert a constant torque on the shaft, and it is turning at a constant rpm, you are generating a fixed amount of power. Turn the shaft twice as fast while exerting the same torque, and you are now generating twice the power.
So power is torque (twisting force) times the rpm of the shaft. At a lower rpm, for the same power you must increase the torque. Remember the rotor needs constant power for constant lift across a small rpm band.
Try this out, hover carefully and precisely in low wind and note the exact torque and engine temperature (TGT, T5, T4.5, etc). It is easy to be precise if you just barely bounce a wheel or skid a little to get the hover precision down to an inch or so. Then beep the rpm down by 3 or 4 % and stabilize at the same height. Note the torque, and engine temp again. The torque should be about 3 or 4% higher, but the engine temp should be the same (because the power is the same, although the torque is different!)
On some model helos, the lower rpm actually takes less power than the higher rpm, and you can measure it this way, note that 3.5 degrees C of T5 or T4.5 equals 1% power on most gas turbines.