Spinning off cliffs in powder is very difficult. The pros make it look like jumping off a park booter; it is not the same. Timing is off, everything is heavy and deep snow always catches the heels when you try to get the spin. With years of figuring it out, this is what I got:
-Take off like a maniac. I dont just mean extending your legs; I mean the spin actually starts on the previous turn. As you begin the last turn, you want to create a "bounce". Drive the tips in the snow and wait for it to pop back out before launching the cliff. This is your lip; that bounce to pop is going to free your heels and you will be free to rotate.
-Patience is key. Hips is where the rotation should begin. Swing the hips and wait for the skis to catch up. If not, the 4 twist motion will set in and a crash is inevitable. If the take off is done right, the skis will catch up with the hips and that is where the grab sets in.
-Dont desperately try to spin. The faster you try to set it, the deeper you sink in the powder on take off. A gaper 360 wont work. A well sequenced, slow 360 is the way to go.
-Practice on bumps, pillows, cat tracks or side tracks; anywhere you can find a powdery lip and heavy skis.