people do them differently man. some people bend their knees and others dont. when i did them my knees were a lil bent on take off, pretty straight completely upside down, and bent again for landing...lol so idk man just do them, and whatever u do, u do...by asking for advice on what to do WHILE ur flipping, you'll be thinking too much, instead of just doing the trick. (if its even considereda trick and not a joke)
http://www.youtube.com/watch?v=L5SLjFRbN9M
http://www.youtube.com/watch?v=L5SLjFRbN9M