Once you get the ball you can't step. Well, technically you can take one step. The idea is to try and keep in front of your player, and intercept the ball as it's going through the air. There is /plenty/ of room for error with balls flying through the air all the time.
You technically can't touch anyone, but there's plenty of shoving. It's just discrete, and almost used as a distraction so they can get away from you and get the ball instead.
*likes watching AFL* *never really got into Rugby* Union or League?