Back layout, land, immediate 1/2 turn.
stomach drop 1/2 turn to stomach drop. start with a stomach drop to stand, add 1/2 turn, eventually connect. Other tramp stuff like this. I'm envisioning back drop to stomach drop, perhaps even training 1/2 off landing the stomach drop till you can do it in the air (and of course being able to do back drop pullover, back drop to handstand prior)
I've also seen some people train the back tuck/pike open and 1/2 out. So turn 1/2 as you kick out just as is commonly trained for front 1/2.