It is a factory-manufactured fin with post-manufactoring alteration (cut off heel). You are supposed to wear it other way around through the hole in the heel. It does give you extra speed this way, but it is quite painful for the top of the foot. all energy comes from downkick, no upkick. extra thick half socks (8 mill or something) would probably solve the issue, have to find those now. 3mm half sock does not work.