What is the purpose of using a wand to hit a monk in Zen meditation?

I have seen Zen monks in meditation, usually with one monk supervising. The monk supervising usually carries a wand of some sort. Every once in a while he would rest the wand on the shoulder of one of the monks and then respectfully hit his shoulder. It did not seem very forceful yet I found this confusing because to me it seemed as if it may be a method of harm or violence. I would like clarification on this practie. Why does the supervising monk do this to the meditating monks?