Oh well, there are a million reasons not to punch that way. But this would only be argumantative. Ideally, you would not throw punches that way. But let's just suppose it happened.... B slapped A's lead hand punch. A's lead hand bended. A's lead elbow followed the momentum and aimed for B's head. Now, while you may use the word "trap", but you are just slapping A's lead hand punch, right? You are not grappling A's lead hand, right? If so, then what is stopping A from using his both elbows and knees? In fact, this is a classical Muay Thai technique when your punch is parried. In Muay Thai, you do not want to over-committ when you slap (or in Muay Thai term, sweep) away an attack. B/c if you sweep too hard and over committ, you just invite the elbow to your face. But, if I read you correctly, you are saying that my hands (A's hand in this case) are "trapped" . How so?