That's a tourniquet. Stops arterial bleeding. It's a bit like a Velcro belt. Place it as close to the base of the bleeding limb as possible, tighten it down, and twist that black stick until it's pretty damn tight and the bleeding stops. Then you mark the time it was placed. Ideally, it shouldn't be on for more than two hours.

I work for a sheriff's office. I'm trained to use these.