alright so i hope this makes sense im gonna try to explain it. when you are going off, start it off as a gainer or backflip but once you start it and start going backwards drop your left shoulder and look over it. you should still try to arch your back at first like a gainer/backflip, and that will help you avoid going into an underflip. i had trouble with it first too, but it helps if you try what i said. hope that helps bro.