Zwerch is the beloved horizontal strike that defends the head without losing its offensive potential. Meyer describes it as coming from Zornhut, but it works equally well from vom Tag or Ochs.
The basic rule is that if you can use a Zornhau, the diagonal strike, then Zwerch is also an option. Practice both equally so that you can call upon either as needed.
Notice how high his hands are. This is essential! If you are not performing this cut above your head then you are not protecting your head.