The Manchu style ring looks as though the string pressure is pulling straight off the large knuckle of the thumb, whereas with rings that require bending the thumb and placing the fore finger over the thumbnail puts the pressure more on the first thumb joint or divided between the two. Am I correct on this, or no, and if correct, which is the stronger method? As of now I have never used a thumb ring. Just trying to determine which way to start.
In my experience, amgakji, without a groove carved into it, put all the pressure on the under side of the last joint of the thumb and all the tension on the middle knuckle of the thumb. The end digit may as well be for decoration as, for me, it's the fingers that keep the digit pressed into the cove of the ring and the ring in place.
Manchu rings, worn on top of the end knuckle, rely on the inherent friction from the fit of the ring on your thumb and the friction created by bending your thumb within the ring. So they perhaps distribute weight more evenly. You don't really feel it though. What you feel the most, assuming you're not using your index to lock your thumb, is having to squeeze your thumb into your middle finger while trying to bend your thumb within the ring. If you lock your thumb with your index the whole thing is much easier.
You can wear them under the knuckle and successfully shoot like that if you stuff some leather up the back to take up the slack. This would rely on friction and the width of the knuckle to stop it coming off. The feeling is pretty much the same.