This technique is used when your attacker throws a straight punch.
When your attacker throws the punch, block the strike with a basic inside forearm block and step with your left foot to the left of your attackers left foot.
Now twist anti-clockwise on the ball of your left foot. Take your right arm behind your attacker and hold their right bicep with your left hand.
Place your right foot to the right of your attackers right foot. Take your left foot further to the left and bend your left knee as shown. Keep your right leg nearly straight with the ball of your foot on the floor. Your calf muscle should be touching the shin of your attackers right leg. Make sure your hips are not in front of your attackers hips.
Now twist your upper body anti-clockwise and put the heel of your right foot on the floor. Your attackers right leg will be swept backwards as your right hand drags them over your right leg.
To finish with an arm-lock, take your left hand round your attackers right arm just behind the elbow.
Put your left knee down on your attackers neck and put your right knee on their chest. Put your hands together and push your right hand to the left.