|Nagamine shinshii's Naihanchi Nidan is very similar to the version in the video below|
I used some clip art to try illustrate these key points below.
The force of the slap is focused (with a heavy feeling) on the opponent's fore-knuckles on initial impact and your finger are allowed to curl over the hand. The palm heel of your right hand and thumb of your left hand press against the back of the opponents hand. As your left hand rotates to apply this thumb pressure it also causes the pink side of your left hand's grip to press against the inside of the opponent's wrist. The opponent's fingers must not be allowed to uncurl. The combination of these causes the wrist to buckle and fold in on itself. Be very careful, the fact that the fingers are forced to remain in a fist (ie hand isn't allowed to open) causes the strain on the connective tissues in the wrist to be greatly increased (as is the pain and potential for injury).