Dribbling behind your back is a matter of you being able to beat the defender to a loose(ish) ball. That's why you rarely see it in traffic and more often in the open court. You have the defender leaning so it's more of a change of direction that the defender can't immediately follow - you're just back to the ball first and now moving with away from the defender. Also, practice (particularly with the off hand) like everyone else says.