It's uh, a tourniquet. Specifically the North American Rescue C-A-T, "the good one."
Here's what they say about it:
The patented C-A-T is a true one-handed tourniquet proven to be 100% effective by the U.S. Army’s Institute of Surgical Research. Tests proved that the C-A-T completely occluded blood flow of an extremity in the event of a traumatic wound with significant hemorrhage.
The C-A-T utilizes a durable windlass system with a patented free-moving internal band providing true circumferential pressure to the extremity. Once adequately tightened, bleeding will cease and the windlass is locked into place. A hook and loop windlass retention strap is then applied, securing the windlass to maintain pressure during casualty evacuation. The C-A-T’s unique dual securing system avoids the use of screws and clips which can become difficult to operate under survival stress or where fine motor skills are compromised.
The Fastest, Safest, Most Effective Prehospital Field Tourniquet
Official Tourniquet of the U.S. Army
Proven to be 100% effective in occluding blood flow in both upper & lower extremities by the U.S. Army’s Institute of Surgical Research
NEW Single Routing Buckle for faster application, decreased blood loss, effective slack removal, fewer windlass turns, and simplified training with single protocol application standards
Includes reinforced windlass, stabilization plate, windlass clip and writeable windlass security strap
Featuring NAR’s signature Red Tip Technology® elliptical tab providing added visual clues during application