It is a fine line on when to intervene in landing with new folks. Especially at first you need to be on your toes and yes applying back pressure in the flare will avert a firm or hard landing and the person may learn from it being debriefed, BUT I still need to see them do it without my intervening. I'm not saying don't intervene if you expect a hard landing, but not every firm landing is hard.
My experience is once a guy has a firm landing, he learns that "hey...ok, I need to pull a little more". You can't just intervene every leg and expect them to learn.
Most the time the guys are looking way to close to the landings and I would bark out at 50' "eyes to the end of the runway!". That seemed to get folks to see the sink rate.