there are a crapload of variations on it. In the pipe, off hips, and off jumps.
I was sessioning with Cusson this past year, and he does them more backflip style, whereas i do them more lincoln loop style.
I found that it is easiest off of a hip. if your good at backflips, then go off the hip and lean back, while looking over your shoulder as if your gonna spin. You want your feet to follow a 'flat' axis, which is the happy medium between the flip and the spin. On a hip, most flatspins look like a rodeo 450. You dont want to get totally inverted like a regular backflip, but also not totally upright like a spin.
Cusson throws a few in the Game that look really good.
For the lincoln loop style, i pop off and do a lincoln loop with 90 degrees of rotation that is spread throughout the flip. I will see if i can find a clip and upload it.
Hope this helps bro!
Spinks