If the cloth is fastened on the string back and pulled over the string at the front, you get a set of underwear that is pretty easy to adjust to size or whatever other needs you may have. http://images.pingmag.jp/images/article/fundoshi05.jpg if my description was not clear.
I learned some new things today. Thanks.
If your link is what the underwear is supposed to be, the cloth should be narrower where it loops over the string. Movement will cause the edges to move inwards, until the cloth bunches so much that the slip no longer, well, slips inward.
I know I'm nitpicking here, but people are bound to have an issue with it. As I do, apparently.
I apologise for being a donkey.

Great mod overall; the underwear just doesn't seem quite right.