To start get into a tie-up position. For example your right hand on your opponents neck and your left hand on your opponents right arm, or visa versa
Also you can push your head against your opponents head but remember do not do it so you guys are ear-to-ear. Instead only the tops of your heads should be touching. This will allow you to see where his legs are, and give you better access for a shot
If you tie up with the example i gave you in step one the you will be doing the duck under on your opponents right side
The next steps should be done very quickly and explosively
Pull up on your opponents right arm
You will step with your left leg and go to your knee on your right leg
At the same time pull forward on your opponents neck to get him to also step/move forward
Along with steps 5-7, duck your head just low enough to clear your opponents arm
When you clear through your opponents right arm then you will be behind him and now can take him down any way you like
Make sure you go very quickly with this move. You only have a split second to act once you pull up on your opponents arms to make room for you to duck underneath. Because obviously when he see's that you are trying to do a duck under he will snap his arm back down and deny you. Then he may keep his arms in really tight so that you dont try to attempt that move again. So go quick
If you dont succeed the first time keep trying. It may have failed because you didn't duck low enough to clear your opponents arm, or your not pulling his arm up high enough for you to get under and around him.
