This technique is used when the opponent tries to grab the defender's wrist.
Start off in the Aikido stance. Allow the opponent to grab your wrist.
The opponent, in this picture, grabs your left wrist with his right hand.
Raise your left arm enough to make the opponent loose the grip (slightly).
Peel off the opponent's hand by sliding the fingers of your right hand between your
forearm and the side of the opponent's palm.
Place your thumb at the root of the opponent's index finger, on the back of his/her hand.
Use your grip on the opponent's hand to manipulate its position, so that the palm of
the opponent's hand faces his back. This can be done by pushing your thumb into the opponent
so that the root of the index finger starts to twist the entire hand.
While doing so the opponent should feel that he/she cannot handle the pressure applied on
the hand, while standing erect. The opponent will have to bend forward in order to
relieve a little stress.
To ensure that the opponent does not use his/her other hand to attach you, you can push his/her
elbow down using your right hand.
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Watching a video might help you better understand the motion.
Lets examine where most of the pain is generated, while applying this technique.