The lateral strike is made usually with a thumb grip, i.e. with thumb on the flat of the blade. It is made from the right shoulder, and travels horizontally to the left, or to right, at about head height.
I prefer to make the strike with my left leg forwards, and strike from the right shoulder Vom Tag position into a left Och without taking a step forwards. This gets me used to generating the power with my core. I then 'follow' the strike with a step forwards-right. The two should happen more or less at the same time, but the strike should lead the step and not the other way around. The power is NOT generated by the step, this is too slow, at least in my opinion.
Too slow for what?
Well, this is where the purpose of the Zwerchau comes into play. It is essentially an attack, or a parry, that is used when the opponent is in a Vom Tag position (or attacking from that position). This could be held at their shoulder, or they may have their hilt lower down but blade pointed high. The idea is that, if using it as an attack, if they make a flinch response attack into your attack, this is likely to be a simple Oberhau. The Zwerchau will block this simple Oberhau either with the blade or with the crossguard. This means you need to get your sword and crossguard forward in front of your hands before they can strike you. If using the Zwerchau for defense, the same is true. Therefore, speed is of the essence.
So how does it work? From a geometry perspective it is quite simple: when my opponent is in a shoulder vom tag, his strike needs to come forward. As his blade is pointing upwards, if I place something - my blade or my crossguard - horizontally in front of his blade, I block the path of his blade (red line in image below).
If I then rotate this red line around the horizontal axis, I can smash it into the side of his head, or get the point online, all the while defending my own head from his blade.
What does the good book have to say about it? Well, the RDL glosses say that the Zwerchau counters all strikes from above, so I guess that includes the Scheitelhau and the Schielhau too. It also says that you should jump with the right foot away from his strike to your left. So, it's just a Zwerch to the left... and a step to the right, like some sort of 15th century time warp dance.
Plays
Here are the main plays of the Zwerch:
Y: You Zwerch to your opponent, who is in vom tag.
O: Opponent parries with a common parry or an Oberhau.
Y: If your opponent is soft in the parry you should be able to thrust him in the face.
Y: If your opponent is hard in the parry then shove your crossguard forwards and wrench his blade to your right, and as you do so rotate the blade around over your head and Zwerch him to the other side of the head (like a helicopter blade).
O: As you come around to Zwerch on the other side of your opponent's head, your opponent can counter this in a few ways:
He presses his blade, horizontally, against your arms.
He Zwerches as a mirror image of your Zwerch and gets his blade over your shoulder, but lower than your blade and hilt higher. Your blade will hit his driving his blade into your own neck.
Y: If your opponent tries to slice your arms as in 5.a, then you perform a duplieren and stab him in the face.
O: As you try and strike the duplieren, your opponent can raise his arms and wind into Ox towards your blade and stab you in the neck.
Zwerch to Ox and Plough
The Zwerchau can be made to the high or low line (referred to as to Ox and Plough in RDL). It basically says: Note when the time is right and jump to him and strike a Zwerch to the low left side, then strike a Zwerch above right, then lower right, and then upper left. If you are familiar with Meyer, this is the Meyer's square pattern.
Now, the counter to the lower Zwerch is a little confusing, as RDL is not very clear on how we are supposed to parry the upper Zwerch. However, it is likely that the upper Zwerch has been parried with the hilt high and the blade low, so some sort of 'hanging Ox'. When the Zwerch is then made to the other side but low, we are told to keep our hilt above our head and to move our point to the low line and then stab them in the stomach or legs. So, from this, it seems that the Zwerch is being parried with a 'hanging Zwerch', i.e. if their blade is horizonal, yours is hanging with the hilt higher than theirs but the blade lower. If you Zwerch into their Zwerch you should parry it. Your point should then be moved online for the thrust.
Failing
This is the part in the Glosses where failing is introduced. It is a simple enough concept: if you Zwerch to their top right and they move to parry it, do not let the blades touch and immediately Zwerch to the bottom left. If they go to parry that, then Zwerch to the bottom right before the blades touch, and so on. It is basically a feint, with the key difference being that if they don't move to parry it you want to hit them in that opening, whereas a feint is usually a 'pre-planned' fail; in this case you only want to fail the strike if they move to parry it.
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