The Chapeau! snap wrap leather trouser strap really is one of the simplest things to use. It's purpose is simple, hopping on a bike but not lycra clad then you are going to be wearing shorts or trousers (or maybe not but that's a different story).
Simply take the extended flat strap, fold over your trousers near the ankle on the drive side of the bike and slap on the strap. It will wrap around the ankle and avoid any unsightly oil marks on your trousers.
Simple and stylish in a Black leather finish