CubanB, it’s held by your buttcheeks & your inner thighs. I kind of use BTC to gauge how my flaccid length is coming along, by noting how much further back I can pull it. Kind of a general form of measurement.
Maybe my buttcheeks and thighs aren’t thick enough. Personally I can see this doable with tape. Of course the hair on my ass and thighs are going to disagree abit :0