Shrugging; it is not something I teach my athletes. I rarely discuss it. I allow it to happen naturally in the body. But a question recently posed to me was whether we should teach our athletes to shrug up (vertically) or shrug back. If you feel you must address it at all I would say shrug up. Teaching to shrug back over-complicates the movement and I prefer to keep things simple (which is why I do not mention it). If you must, shrug towards the ears.
Trainers who encourage athletes to shrug back may be trying to aim the bar back towards the shoulders (Clean) or behind the head (Snatch). Or they may be making the effort of creating more elevation. But what pulls the bar back in place to receive it isn't shrugging but rather leaning back and the aggressive use of arms in the turnover.
Shrugging alone is too short of a movement to affect bar’s trajectory (path). You want the bar to go back? Aim it with the full body (legs, trunk and arms). You want elevation of the bar? Again, use the whole body, primarily the legs.