This is a high cut with the short edge of the sword. It is used to attack over an obstacle such as a sword in Ochs or a shield. In the SCA this is known as a Scorpion or Sky Hook.
Although this cut is a High Cut, and so considered because there is not much difference between the two, yet this is called the Plunge Cut because in cutting through, it always plunges over above, so that the point comes against the opponent’s face in the Ox; and it is most used in the Approach or Onset.
If you want to skip ahead, you can see an example on page 36 of using the Sturtzhauw to transition into Ochs and from break your opponents Ochs.