In the Liechtenauer glosses we often see this passage,
The second is the Zwerch [thwart-cut], which breaks the guard vom Tag [from-the-roof].
This is one of the “vier versetzen” or “four displacements”. While the word displacement may mean parry, in this context it means to cause someone to leave their current posture or guard. This is also referred to as “breaking” a guard.
Breaking a guard is important because it is very risky to directly attack someone who is resting in a guard. In theory they know how to counter any basic attack against them, otherwise they wouldn’t be waiting in that posture. (Excluding, of course, someone who is fatigued and is literally resting in order to catch their breath.)
If we accept the theory that someone is waiting in Tag (or vom Tag if you prefer) because they believe they are safe from simple attacks, it stands to reason that one cannot simple step into range and hit them with a Zwerch. If it were that simple, no one would ever rest in Tag.
Meyer’s Opening: Threaten a Thrust
In his fourth device for Tag, Meyer tells us that our opponent has chosen to linger in Tag. Being in Tag ourselves, we are to first turn our point towards the enemy with crossed wrists. In such a manner we threaten him with a thrust.
This should be done at such a range that, if your opponent ignores the threat, you can complete the thrust with a step. Depending on the measure you may use either an advancing step or a passing step. Either way, if your opponent waits until you start the thrust to respond chances are you’ll hit in a way that leaves you safely behind your blade.
But lets assume he reacts as soon as he see the point near his face…
Against the Beat, Zwerch
If the opponent acknowledges the threat, he’ll have to do something about it. The most likely response is to bat away the sword to your left, his right. Allow this to happen, as it will just give power to your Zwerch.
I suppose he could try to beat your sword to your right, but with crossed arms it is like to barely move the blade and leave open the option to thrust.
Against the Cut, Zwerch
Instead of attacking the sword, your opponent may try to step offline while cutting at your head. If you sense that he is going to do this, start the Zwerch as soon as he begins to move so that you can parry the cut and strike him at the same time.
Social rules in Germany included a prohibition against thrusting at other Germans. Likewise, fencing in a tournament or school setting may prohibit the thrust for safety reasons.
In this situations, the threat of a thrust will still usually have the desired effect. Some will be stunned momentarily by its presence, others will seek to bat it away. It takes a lot of nerve to see the point just before the face and consciously ignore it as a false threat. (Especially if you consider that your opponent may be ignorant of the social rules or simply sloppy and dangerous.)
Many of us have long complained that fencing masks dull the senses and dampen the flinch reflex. And while I still maintain that is true, it is more of a problem for cuts that thrusts. Because the point is coming straight at the opponent, he can see it the entire time and there is only a minor difference between using this technique with and without masks. Mostly this means you have to be somewhat closer to instill fear.