Your Collective Pitch and Throttle need to be separate channels because they don't always act together in the same fashion. For aerobatic flying (or I guess just added stability) you have a flight mode switch. Generally you'll set up an Idle-Up mode on this switch. When you flip it the motor RPM will rise significantly, depending on the setup it may just stay at a constant speed near the max for the motor, and your pitch curve will change, putting 0 in the middle and giving you an even range of positive and negative pitch to allow for inverted flying and more maneuverability in general.