From OpenwaterpediaEney Jones, especially suitable for training for open water swims. The EneyBuoy is a blow molded plastic buoy comfortable with ability to adjust the buoyancy, excellent for open water swimming training.
U.S. Patent 4,929,205
It is an adjustable buoyancy structure to be held between the legs of a practicing swimmer in order to assist in developing arm muscles while swimming. The drag is also beneficial to swimmers practicing the butterfly, backstroke, breaststroke and freestyle swimming stroke and functions to substantially immobilize the legs of a swimmer against movement relative to each other.