This technique is used against an upper-cut punch.
As your attacker throws the punch, step forward on your left foot and block the strike with a left handed downward block.
Use your right hand to cup round the back of your attackers right elbow.
Pull your attackers right elbow out to the side keeping your left arm in contact with their arm.
Place your left hand on the back of your attackers right elbow and take your right hand away. You can now finish with a right knee strike.