Once upon a time… or so these kind of stories go. This style of ring has been seen in Narnia, and The Rings trilogy. Unfortunately, due to some misinformation on the interwebs, most folks wear versions of it backwards. This would destroy the ceremonial designs on the face and send gems flying as the bow string crossed it
The ring is designed to give mechanical advantage, and not to protect the thumb as so many folks seem to think. These archers had calloused hands like a guitar player from practicing daily. They didn’t need to protect their thumbs. What they needed was rapid fire strumming. The hook at the end was used to grab the bow string and draw it. With an upward flick of the wrist, the arrow would take flight, the archer would draw the next arrow from the quiver, and then knock it in one graceful motion.