Why is the non-dominant arm kept behind the back in messer fencing

Why is the non-dominant arm kept behind the back in messer fencing? I’m not suggesting the non-dominant arm should stay glued behind the back. Lecküchner uses the non-dominant arm a lot for things like disarms, half-swording and wrestling, but when the non-dominant arm isn’t in use it is behind the back in pretty much all of the illustrations. Why is that? Let’s start with a comparison with sabre. Sabre guards vary from system to system, but are typically done with the dominant foot forward, the dominant arm extended and the non-dominant hand on the hip. From the front my right hand can’t even be seen, as the sabre basket covers my hand and part of my lower arm. The left arm is behind my body, so that arm isn’t a viable target. What happens when I do the same thing with a messer? While the crossguard and Nagel provides some protection, it is much less protection than the sabre basket and my right hand and arm are very exposed. The first rule of fencing is “Don’t get hit!”. The second (or at least near the top) rule is “Anything that sticks out will be hit”. With a messer staying in this position is a terrible idea. Messers will cut well even near the tip. Lecküchner’s manual even has an entire section (called Lemstück) for attacking the wrist. Let’s fix this by retracting the right arm and right leg. Now my right hand and arm are no longer unnecessarily exposed. But we’re not done yet. Leküchner’s messer fencing uses positions both with the right leg leading and with the left leg leading. With the left leg leading, keeping the left hand on the hip causes my left elbow to stick out. In order to eliminate the left arm as a target, put the left arm behind your back. Now the left arm is denied as a target both when the left foot is leading and when the right foot is leading. Almost all of the illustrations in Lecküchner’s manual shows the left arm behind the back. The text also mentions the option of keeping the left arm across the chest to be better prepared for wrestling. Lecküchner's langes messer guards Lecküchner's secondary positions Images from Cgm 582 are courtesy of Bayerische Staatsbibliothek.