Snow - a completely inverted on-axis backflip with a 360° twist thrown is is called a back-full, as said above. Old-school freestylers also called it a mobius loop.
On a tramp, to get it really smooth, you must first be able to throw a laid out backflip. I'd also recommend that you know how to do a barani comfortably (front flip with a half twist). You'll see why later.
Here's the trick. With your arms above your head, throw with enough power for a laid out back. Turn your head to the left (assuming you twist left) and LOOK under your left arm pit. You should be able to see the tramp. Drop the left arm to your side (about a 1/3 into the flip). Believe it or not, you will twist (tilt-twisting mechanics). When you're completely upside down, the rest of the twist will finish just like a barani (provided you flipped with enough power). If the twist slows down, bring your right arm down and across your stomach at this point.
You should always be able to see the tramp once the trick starts. Get a spotter or a harness to be sure.