I read through all of the posts and while several people have mentioned some good tips, I'm going to disagree with some of the information presented.
I'm siding with Stephan here in feeling like this trick is easier the bigger you take it. A good wide, hard edge and a solid pop off your tail should be your first focus. I don't like using a progressive edge on this trick unless I'm going to let it drift behind me. Keep the cut a little more consistent and it will keep your axis vertical and on balance.
Now, there are several ways to skin a cat and some of the explanations above might work for some people, but in my opinion you absolutely should be twisting at your waist for this trick. If you don't twist at your waist, then it's going to take a lot more effort to make your board go backside, and then bring it back.
The body position of an indy shifty is almost exactly like the body position in doing a backside lipslide.
Try this: Go do some backside lipslides or frontside boardslides on the lip of the wake next time you're out. Really try to get that board perpendicular to the boat or further. That position is the position you should ideally be in when doing an indy shifty. To execute it correctly, you should move your handle over by your front butt cheek and have it really low. Your legs and hips should be twisted backside. Try to keep it close, but it doesn’t have to be so close it’s touching your body.
Your upper body shouldn't be doing as much as your lower body. I recommend looking in the direction you came from instead of either toward the boat or away from the boat. Looking where you came form will keep your shoulders in the best position (parallel with the direction of the boat). Do this drill several times as it will get your body comfortable moving into that position and help with the muscle memory.
Once you’re comfortable getting into that position, I would say it’s time to take it to the air. After you take your wide, hard, but fairly even cut and shove your back foot into the wake to get as much height is possible, think “frontside board slide” with your body. Almost everything I talked about with body position earlier is going to be the same once you get in the air, with the exception of this: In the air, you will want to bend over at the waist (to get the grab) and suck your back leg up (to help get the grab and to make this trick look good). While you suck your back leg up, you should be getting the grab about now and once you get a hold of your board, that’s when you lock your front knee to get a solid poke.
After that, the rest is pretty self explanatory. Hold the trick as long as you can, let go and bring those hips back around and get your legs extended and ready for impact.
Note* taking the indy shifty to a backside 180 does require less twisting and more rotating, but that’s another story.
Here is a picture of how I do an indy-shifty:
Another for better perspective: