A false edge cut to the head from low left position. The only example of nucken in I.33 says it should be aimed between the opponent's sword and shield. It is a recommended action for binding above and on the left, and it can take advantage of stichslack that the opponent should be doing in such situation.
It is intended to be a fast cut, possibly not very powerful, but certainly surprising. I.33 reccomends us to do nucken very quickly and immediatelly remove the sword. Because it goes between the opponent's arms while not being very powerful, the opponent might then be able to grab the sword with his arms and hold it, which would lead to grappling.