A couple things come to mind on this. Having the student explain their thoughts on impact positions and what they perceive is happening during the impact interval. Are they trying to “help” the ball into the air by flipping their wrists at impact? It is usually a good discussion and leads to my explanation that the club works in a 3 dimensional manner – in the backswing the club travels back, up and inward (inward from the extended target line) and the downswing is down (usually not enough down) out and forward. This can help if properly explained and understood.
Another way is to explain / demonstrate by using an impact bag and discussing the merits of a flat left wrist and bent right wrist (for RH’ers) at impact. I have had great success with both discussions / demonstrations to the student. Since the club is traveling so fast during a normal swing it is always good to feel a static position and capture the feel/look of the correct position. Let Mechanics produce and Feel reproduce as Homer Kelley would say!