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?
Supposedly it is used to help meditator fight drowsiness. It is said that drowsiness comes from stiffness, from meditator trying to artificially restrict their micro-movements and to severely narrow their scope of attention. While right concentration is said to feel open and flexible, with body actively balancing in meditation posture and attention wide open (but not engaged nor scattered). When supervising monk sees somebody nodding, he reminds meditator of the need to expand their awareness, by gently "massaging" the shoulder with the stick :)