It`s like hell, specially on the feet. Perhaps, that because on the skin of the feet you can puch the pencil down even stronger and to make tickling, if you do it like this on ribs for exeample, you will got the paine more than tickling, but if you writte or draw slowly and lightly on the skin of for ex ribs, back, it`s a tickling but not as intensive as on the feet.
It could be compared by the tickling technic than someone peak a place and continue poking nonstop for a while.While drawing or writting on someone feet you are doing it fast, as on the paper, without the ticklish feeling, and finding lee reacting like crazy you would be forced to continue, for sure.I`m remembering that kind of tickling even as one of the most torturous and efective ways of tickling at all.