With the pre-ripped and pre-torn "designer" jeans that kids these days wear, this drown-proofing tactic will soon be lost to civilization, except in the memories of us old folk. I remember learning it in Boy Scouts. I also remember learning to blow air into a tight fitting shirt for flotation help as well, but the jeans technique worked a lot better as I recall.

These days, I would probably drown while trying to get my jeans off though...