This technique is used to pin your attacker on the ground.
Sit next to your attacker with your back against the side of their chest.
Take your right arm and place it round the back of your attackers neck. Grab hold of either your gi or theirs to prevent them pulling their neck out.
Take their right arm under your left arm-pit and pin it there with your left arm. Grab hold of the back of their right arm as shown. This is the strongest grip to use to prevent your attacker from pulling their arm out.
Now tuck your head in and spread your legs as far as possible. This gives you a good base that makes it difficult for your attacker to move. Try to always keep your weight on your attacker.