I did this at the big ramps map, crouching with both sticks while rolling down the roll-in of the mega ramp - before releasing and popping your trick, hold either LT or RT to 'wind up'. When you release both sticks - keep hold of your selected trigger until your character completes a 900 degree rotation.
A 900 is 2 and a half full spins (so 2 x 360 and 1 x 180) meaning you should roll away in fakie. You have to complete a 900 for this achievement to unlock, and it wont if you do more such as a 1080 or a 1260.
I rolled away on the flat ground to the left of the mega ramp rather than trying to hit the gap which worked just fine.
Here's a video I did showing how I unlocked this one: