Pigtail

British kayakers (at least) call these Cow's tails, although they aren't quite the same thing as a climber's or caver's cowstail.

A short towing device. Typically made out of a length of tubular webbing with a shock cord threaded inside to absorb shock. Loops at each end permit the addition of carabiners. This should never be improvised! A pigtail should never be attached directly to a PFD only to a rescue PFD with a quick release chest harness so that the tow line can be disengaged from the PFD if the paddler gets into trouble.