Sorry I never saw this earlier... I learned these way way long ago before inverts were cool. However I was not only an instructor but the terrain park manager so I MADE myself a SUPER soft landing with a snow gun and worked on these after hours in the wee hours of the night.
I had a full write up on these at snowboardaddicts, I'm pretty sure its still pertinent. first here is a video of some very old footage of me doing these in slow mo for one of the old members of SBA. https://www.youtube.com/watch?v=fdoPlNvJsIs
1. think of them as a cartwheel not a flip.
2. commitment is key and KEEPING YOUR EYES OPEN.
3. does your local hill have an air-bag? If not find one or a foam pit, anything other then learning them on snow... I got 3 class A concussions from learning these. (they didn't have helmets when I started riding and doing these)
4. Still want to learn them the hard way? OK. You must make sure on the take off that you do not start too early or late off the lip. right as the tip of your board crosses the lip is when I start these.
5. I drop my lead shoulder hard and slightly push off my rear foot. If you have enough speed the jump does the rest... that's what its there for. The only time you need to nollie is if you are doing these off of catwalks or need the rotation to be EXTREMELY fast.
6. if you watch the video (again this is REALLY old footage) you will see I try and reach down to my board. This maintains the tuck, once you start extending the rotation stops dramatically.
7. While tucked I'm looking over my rear shoulder, because when you are upside down this is where you will spot your landing. If you see the ground from the half way point its easy to see where you need to extend to stop the rotation.
Once I got to this point I started adding grabs and leg pumps and other shit... but it CHANGES the rotation speed depending on where you grab and such. So keep that in mind.
I've done so many of these they have honestly become one of my "safety tricks" for testing jumps... its strange I know but again when I first started doing these... it was rare for anyone to be trying this shit on my hill... hell the biggest trick back then was a misty 5. Let me know if that helps and I'll try and dig up the write up I did on these years ago. I think the best part about learning these as early as I did was it allowed me to be REALLY confident in the air when off axis rotations and flips became popular. Now I feel more comfortable off axis then I do on it.
BUT there is still nothing more epic then a HUGE flat backside one... the control needed for those rotations are just awesome!