Maebukuro

An ecchu fundoshi I made with cotton gauze fabric and patterned trim.

Source: https://fundoshi-doshi.tumblr.com

I’m guessing you’re looking at this picture. There are three type of fundoshi. Rokushaku is the classic one, which most of the posts depict. It’s tied using a simple long cloth. The other two types, Etchū and Mokko use less cloth, but have tapes attached to them that tie at the front (Etchū) or at the sides (Mokko).

L-R: Rokushaku, Mokko, Etchū

An Etchū fundoshi is T-shaped. You tie the tapes (the top bar of the T) around your waist, so that the cloth (the vertical bar) is hanging behind you. Then you pull the cloth through your legs, and tuck it behind and over the tied tapes, leaving that little square flap at the front. This is almost the same design that Romans used for their loincloth, called a subligaculum (literally “tie up your arse”)

Tying a Roman subligaculum

Mokko fundoshi has a shorter cloth with tapes on both ends.

That pic looks like it’s different way of tying a Rokushaku fundoshi, making it more like an Etchū fundoshi.

You take a Rokushaku fundoshi and tie it around your waist with the knot at the back. Then bring the cloth between your legs, and tuck it over the “rope”, like this: