I've used this at work. I think I even have one or two sitting on my desk here at home. The idea is that it's supposed to limit blood flow for distal finger injuries for wound repair purposes.
In theory, the idea is great. In practice they're a little difficult to place. They'll catch the proximal edge of the wound making it difficult to actually place the ring on the finger without causing further damage. Most of the time, if I have a distal finger or toe injury where I want to slow the blood flow for wound repair I'll just grab a tourniquet from an IV start kid and tie that off, complete the wound repair then take it off.