if your landing on your back your throwing it all wrong. i cant really explain how to do it. i just fucked around for a while a eventually figured out how to do them
Try doing fives really quickly but consistently off a small jump (10ft table), when comfortable doing fives super quick take it to seven, that will get you rotating your body as one comfortably, To cork it, look down a little more over your sholder and let your hips come out in front of you. You dont have to force it other wise you will underflip or Dspin and proabably crash.