Peter,
This is a late reply, hope it can help you and whoever is trying to find out more about the Manchu thumb ring.
The Manchu thumb ring release is like a trigger.
When the string being held by the ring, the ring/thumb is pointing horizontally and to your right.
When talking about "rotating your wrist", it is assuming your hand, then thumb also rotate in the same structure, maintain the same structure. So that the ring will not be at the squire position with the string anymore. The string can't be hold by the ring when the ring is tilted upward. The string will start to sliding off the ring.
Also since the ring will be hold by your thumb pointing somewhat upward, which will not be on the same direction of the string movement. That will keep the ring from sliding off your thumb.
If you look at that image I uploaded, the top tow drawing shows holding and after the rotating viewed from the back-right. When the string slides off the ring, your thumb is pointing upward. That holds the ring. (upward - just enough for the string to slide off)
