OYA UBI SHIME or Thumb Grip.
Suppose you catch B's right thumb round the waist, just below the first joint, holding it tightly well down in the crotch between your right thumb and first finger, pull and press the ends of your fingers down into the base of his thumb at the back of the hand, he will probably drop to his knees, to save his thumb. See illustration.
Remember that in a rough-and-tumble fight or scuffle, when you can get one of these grips on your man, he is helpless. Always "look pleasant" and keep cool. This is often half the battle.
From Jiu-jitsu: A Comprehensive and Copiously Illustrated Treatise (1904), p. 13, by Harry Hall Skinner.