Mongolian Draw and Release
The Mongolian draw uses only the thumb, the strongest single digit, to grasp the string. The index and/or middle fingers close over the outside of the thumb to reinforce the grip. This is often called the "thumb draw/release"; it is traditional for all ethnicities across the Asian steppes, extending to Korea, China, Russia, Persia and Turkey. It was also used by Ishi, the last of the Yana, with his short bows.
It gives a narrower grip on the string, as only one digit is used, and this may help to avoid "string pinch" with shorter bows such as the composite bows normally used from horseback. It may also avoid a problem occasionally faced by archers using the Mediterranean release, when the three fingers do not release at exactly the same time and thus foul the draw. It also positions the string further back in the hand, allowing a longer draw, all else being equal. This release is normally used with the arrow on the right side of the bow for a right-handed archer, and on the left side of the bow for a left-handed archer.
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