Idle-up is basically a switch that will keep the throttle at 100%, no matter where your stick position is on the collective. Normally when you raise the throttle on the controller, you are increasing throttle and increasing pitch angle at the same time. When you lower the throttle, you also lower your pitch.

When flying inverted you would enable idle-up so that when you are inverted, you would be able to have negative pitch (sitck lever is in the low position), but still have full throttle.

I'm not sure if I made any sense with this explanation......