Hello!
If I am understanding you correctly you are referring to the movement known as pak sao, which is a hand immobilization (trap). The reason this contact occurs is that when you deliver a lead hand strike, the opponent will often answer with a lead hand defense, in which case pak sao (slapping hand) with your rear hand is used to remove the obstruction so that you may continue on the way to the target. You trap the part of the arm right at the elbow joint, often slamming/pinning it into the opponent's ribcage. The opponent's lead elbow id the key to controlling the centerline. If you control the lead elbow, you control the centerline! This trap, if done correctly, is as much an attack as it is a trap! In other words, the trap itself HURTS the opponent's arm! Pak sao is executed with constant forward pressure, so that your lead hand flies forward when freed up! This increases the impact potential, as well as the directness of your lead hand strike!
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