I don't have this particular TX, but many TXs have a "hover" pitch and or throttle switch on them. This is used for gas helis where changes in barometric pressure or temp during the day can cause the engine to run more lean or more rich with resultant power output changes from the motor. It's a quick way to shift the entire pitch or throttle curve up or down a few points to fine tune how the heli hovers as the fuel mixture changes rather than having to go in and re do the curve each time.
It's not a needed function on electric helis and will actually screw you up if you mess around with them. You can end up with situations where one curve doesn't quite match another using identical values or you can't get 0 throttle even at the lowest point on your throttle curve.
This is assuming of course that I'm right about what this hover function on your TX does