score:8
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 :)
Upvote:2
It's a remedy for sleepiness. Nothing special. It's a little pat on the back to keep you awake.
Upvote:5
During a weekend retreat I attended at a Zen monastery, the explanation I heard was that the sensation provides a point of focus for meditation. At this monastery (or at least at this retreat, which was for beginners), receiving a hit from the wand was voluntary; I don't think they used it to wake up participants.
Other sources (including other answers to this question) have suggested that waking up a dozing meditator is another use.