I too have experienced being held on the floor with a lock of some kind, and a finger in TW17. Yes it is effective. I don't like to use this technique in practice because I get concerned about risk of damage to the structures under it. (Auricular artery maybe? Help?) I used to practice with someone who had to win no matter what. That puts a lot of responsibility on me because I knew he'd rather die than tap out.

It is different pressing into this point in the standing situation when you are using it to turn someone's head to set up a strike or a take-down because with the pressure, uke moves his head and that is the action that protects the point from damage. When uke can't move, then it is more dangerous.
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The older I get, the better I was!