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