SIX said: I dunno, some double sided tape (that's what I'm guessing it is) is pretty tough. So you never know.
It is, according to anko2481 (Yu-Improvisation, by D.O). The illustration is exactly what's going on: Big bro anon improvised a quick torture setup for the reimu sisters. He was even a little impressed that they managed to last 20 minutes before the elder reimu collapsed from trauma of the repeated tortures and bean paste loss.
yukiyuzen said: That nail must be made of some crazy dense material if actually manages to hurt the ko.
I didn't mention it, but anon also coated the nail with hot sauce/mustard (not sure why Google Translate went with mustard when the kanji used in the passage is the same for sesame oil, as far as I know). So even if it won't penetrate the skin, skin contact will be enough.
The anon's objective isn't just abuse, but slow agonizing torture.