Hey Bob. The throttle HOLD switch does a couple of things. Firstly it will cut power to the motor. The other aspect is that it will also move the swash plate to full negative assuming the throttle stick is at zero (down). You will still get full movement of the swash plate as if you are in Stunt mode (full positive and full negative pitch) but no power.

HOLD should be used when you are changing batteries as a fail safe for example. Other times to use throttle hold are for auto-rotation practice or if you are about to crash!! If you can flip the HOLD switch quickly you remove the motor tension from the main gear and might save a few things!! You have to be quick though.

HOLD tends to be used a lot on the bench for setting up pitch angles etc. For example at mid-stick (50%) on the throttle you can set your zero pitch angle when in HOLD. CAUTION though... if you switch hold off when still at mid-stick the blades will spin up. Most of us do this set up in Stunt Mode (referred to as Idle Up) with at least two motor wires disconnected.

I hope that helps.

Jon