Check out the latest rapier demonstration showing parry techniques with an off-hand weapon. Greg Hinchcliff demonstrates fighting first with one weapon vs. two, then switches to two weapons against his opponent.
In this chapter Meyer introduces a key principle. This is in chapter 10 of his treatise incidentally; and a curious place to put it. Apparently Meÿer’s idea of study was to consume his treatise whole because the preceding techniques cannot be applied with cohesion without this and some other particular sections being adhered to as an overarching doctrine or methodology.
The idea here is that all preceding stances and strikes should be approached in the context of only these four openings and in such a manner that the vor, mittel and nach moves are copacetic to each other.
My interpretation of this diagram follows:
Looking at the upper right quadrant and the outermost rectangle we start with the first opening or strike that comes from this part of our own body; following the red arrow, this would be executed with something like an overhead/vertex strike or oberhau. Since the cut takes us across the body we will find our stance and footwork naturally taking us to having our weapon point at the lower left opening of our opponent. We then re-position our weapon with the appropriate step forward or to one side along with moving our blade quickly to the 3rd opening on the outer rectangle and thus we would execute an unterhau from something like the changer stance and end up in something like left Ochs or Unicorn and arrive at the 4th opening.
The other sets of arrows simply indicate alternate starting positions depending on how the Fencer wants to begin.