Well, Its a completly different ballgame when your spinning onto urbans. First of all, you want at least twice the speed you would get for a normal 90 on or a 50/50. So you want a solid slingshot from around 4 or 5 people. Just tell them your gonna spin and they should help you out.
As you approach the jump, you should be leaning quite far back so you dont catch your tips on the rail. Put your arms out to the side so you dont lose your balance. This also shows homies that you mean buisness. Maybe even go into a slight snowplow for traction. As you get onto the jump you want an extremly narrow stance. Make sure you are still leaning back, but keep your feet glued together. This will alter the transfunctional mobility of your bases on the snow, giving you that extra little "hapsst" to your trick. When you hit the lip, you DO NOT want to look at the rail. Instead, I like to spot right above my head. Really whip youre head in this upwards-sidways direction using your neck and poles to make sure that your rotation is clean. Pop off of the sidewall of your skis to minimize surface friction.
From here, GET THE GRAB. Safetys, mutes, and true noses look dope on urbans. Alot of people are scared to commit to the grab, but remember that you have more speed than normal, and that extra moment to get a solid grip is always there. Once you are confident you have the grab, rely on your in-air intuition to get you onto the rail. Let go of the grab, and land gracefully. Most of the time, I am unaware of the rotation i just did. sometimes its 450, sometimes 270, Ive pulled a few 630's before but i didnt get the grab. When you get up to that level however, the grabs dont matter...as much. On down rails, I find that your balance is often forwards, so I like leaning back a little more than i think is appropriate to avoid catching my edges, and falling forwards.
From here, it doesnt really matter. You dont need to "clean" or "grease" the rail as they say in the buisness. Computers are a wonderful thing, and can easily edit you sliding the rail to the end. My balance is usually strangley off centered within the first few seconds on the rail, so I safely slide off into the stairwell. Rest assured this is the "safe zone", and there isnt to much that can go wrong at this point. Maintain the principles of the inrun during this stage and you should be good!
I taught tom wallisch how to do these during an elbow down sesh in india. Follow this procedure and practice it, and you should be golden for a solid video part in level 1's next movie.