Usually a piece of leather, that is attached to the waist, that runs down and under, covering the scrotum, running between the buttocks.
The front portion of the strap can be split, to allow the penis and balls to not be covered, restricted.
It can be closed, so as to force the penis and balls back into the body.
Is also used to insure that an inserted butt plug / dildo is kept inside a submissive's rectum.
Some commercial forms include the butt plug attached to the rear portion of the actual strap. Additionally some straps allow for the attachment of a butt plug.
Most straps are attached to a harness, or some other device in order to keep it in place.
Was originally included as part of a Straight Jacket, to further hinder escape.