Sure there are pass patterns where a QB judges from the positioning of the defense and the cover man where to place the ball, but that's not what people are talking about with "back shoulder throws". They are talking about called plays where a receiver sprints downfield in an effort to get the CB to turn and run with him, knowing that he is looking for a throw to the backside while the CB is running with the expectation of a deep downfield throw, thus his momentum takes him out of the play. It doesn't take an elite receiver to run that play, but it obviously does help if the CB thinks of the receiver as a true downfield threat. It also takes very good timing and judgement on where to locate the ball - the receiver and QB have to be in sync.