"Mostly novelty, and nothing too complicated about it.  It's a simple valve on the bottom.  The builder brazed in a bung for the valve, then brazed a watertight piece of sheet stock below that, inside the tube.  The upper hardware is standard and used commonly for mounting water bottles. There may or may not be a watertight seal at the top - the junction with the head tube would be watertight anyway. @Juan Cano - stopping to drink is the point for a lot of us.  It's more about socializing than getting our heart rates up.  If you put the liquor in the top tube, you're raising the center of mass a lot, which will affect the handling of the bike.  In the down tube, it ensures as much of the liquid mass as possible is always as low as possible.  The mud is a real concern, but if you're an off-road cyclist, you're used to a little mud on your water bottles, and cleaning the valve off before pouring wouldn't be a big deal.  "