Being hit by a bamboo shinai on bare skin really, really sucks. It hurt more than being hit by a wooden bokken, and considerably more than one would expect really. The reason for this lie it the Shinai's construction. Made by bundling bamboo lathes together, the shinai expands at the point of impact, forcing the lathes apart. As it pushes harder against the victim's skin, however, the lathes compress back together, tighter than they were before, with the victims skin/flesh caught between them. This tends to pinch like all get out in addition to the pain of impact, and leaves angry red lines on the victim's skin. I know this as there was a masochistic bastard in my dojo who would willingly take a solid shinai strike across the stomach once a year as a ritual.