If you pull the throttle stick all the way back to zero, that generally means to stop the motors in LP. That is a safety feature. You don't want to have to think "Now how do I stop the motors?" or "Where is that switch?" if your quad is about to hit someone.
First of all, you need to have your motor neutrals and ESC calibration set correctly, but I assume you already have that.
There are four ways I can think of to reduce the throttle when inverted when doing flips:
- Manually move the throttle to a little above zero. Difficult because sometimes you might set zero (and motors will stop) and other times you might set more throttle than you want which can pull down to the ground more than you want.
- Adjust the transmitter throttle trim so that zero throttle stick still moves the motors. LP does the rest. It will automatically greatly increase the motor speeds when stabilization or command asks for it; as long as the motors are still being commanded to spin. Adjust the trim after arming for each flight where you want this feature. Adjust the trim back to allow stopping the motors and disarming. Trim must be reset back to neutral or you can't arm it.
- Set the "Motors always spin / always stabilize when armed" setting in the output page. You really need a disarming switch for emergencies when you set this up.
- Use CruiseControl on throttle and leave the throttle stick set to mid to high throttle during flips. The default for this is to reduce the motors to 5% when inverted (actually once the quad is past 90 degree bank (105 actually)). The default for CruiseControl is also to increase power automatically when you bank the quad; to increase it the correct amount to maintain altitude in a turn. This boost can be disabled if all you want is to automatically reduce to 5% when inverted. Zero throttle stick stops the motors as expected. Settings are in Stabilization->Expert IIRC.
CruiseControl with Rattitude is the recommended way to learn flips.
I regularly do flips using CruiseControl starting from about 1m, giving full throttle and leaving it there for the whole flip, and doing the flip at about 3m with the initial upward speed. It really couldn't be simpler. This is on a medium powerful quad (not high powered) with flip rate set to 360 degrees per second.
Back off of insane configuration for your flips when you are learning. I like 360 for rate (one flip per second) to start with because most people can keep up with it while it flips and you can mentally imagine what a 1 second flip would be like, count by seconds and imagine 1 smooth continuous flip per second. This 360 flip rate avoids you doing a flip and a half when you only wanted one flip, but you do need to have a little more altitude or upward speed when you start the flip.