This technique is used against a straight punch.
As your attacker strikes, step forward and to the left with your left foot and deflect the punch with an outside parry.
Place your right hand on top of your attackers arm as you walk round to their back.
Now grab your attackers right shoulder with your right hand as shown.
Reach round the front of your attackers neck with your left hand and grab the right side of their collar with your thumb on the inside. Now pull your left elbow back to apply the choke.