Thanks Pilolight for the indepth explanation of this. I learned more about this topic as well.

Yetti: If you'll notice when you unplug your + main motor wire and unplug the tail motor, You can flip the idle up switch and see the drastic change in blade pitch even with the throttle stick pull all the way back. What happens is it puts so much negative pitch in the blades, even though they are still spinning the same direction it actually reverses the way the blades lift is directed. So with the heli sitting on the ground you flip the idle up switch on with the throttle pulled all the way back and the blades spool up, you can actually put your hand over the heli and feel that instead of the air moved by the blades being directed towards the ground, it pushes the air straight up which results in pushing the heli towards the ground. Now if you are inverted it will push the air towards the ground keeping you off the ground.

Like Pilot light said when your idle up switch is flipped on, At half stick on the throttle the pitch should be 0 which means the blades are not producing thrust in either direction. When you want the heli to hover inverted you actually pull the throttle stick towards you to reverse the blades thrust. Also called negative pitch.

Hopefully you can understand that Yetti, as I am not as good at explaining it like PilotLight is.