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.
The older I get, the better I was!