And now we come to the last movement in the naihanchi kata - the double side punch. Yes, I know it's not really two punches to the side. Well, I guess it could be, but that would be a kind of obscure application. A more common explanation would be holding something (like an arm) against your belly with the bent arm as you smack it with the straight arm.
My favorite explanation for this movement is simply a continuation of the hammerfist idea. Earlier in this series we saw that you could do effective hammers straight forward, or to the corners, either with or without dramatic weight-dropping shifts. Here we see one more angle for an effective hammer - horizontally to the side.
Additionally, to my way of thinking, the position of this technique within the kata (at the end of the line) suggests its importance and its use. I've said earlier that I view all of the techniques in naihanchi as very general-purpose motions useful in a close-quarters fight. These techniques can be applied in the kata ordering but they can also be effecively combined out of order counteless ways. When you get into this type of situation you could almost start blindly flailing your arms in naihanchi patterns, and end up improving your situation. Well, when you're in the middle of your naihanchi flailing, if you get confused or lose your place... go back to the end of the naihanchi line and start over with either the backhand or the double side punch. This gives you a pair of effective backups for when the rest of your ideas from this kata start going south.