In most of Asia, especially the Mongols, Turks and Japanese, they used a thumb release. The thumb tip hooked over the string and the fingers folded down over it, to hold it in place. To release, they relaxed their fingers and the string flipped the thumb out of the way. So, they made thumb rings to protect the skin, just like we use gloves and tabs.

They made them from leather, bone, ivory, jade, silver, copper... I've seen pictures of some really beautiful ones.