Continuing with Liechtenauer, we see this passage:
Listen to what is wrong,
do not fight above the left, if you are right
and if you are left, on the right you limp as well.
My first guess was, “If you are right handed, don’t use high guards on the left. And if you are left handed, don’t use high guards on the right.” But that really doesn’t sound right.
If you are right, as in right-foot forward, then you shouldn’t use high guards on the left. Likewise, if you are left foot forward your steps will be clumsy if you are in the high guard on the right.
This seems to correspond to the general idea that you throw cuts from the right side by taking a step with the right foot and vice-versa. However, it isn’t hard to find a counter-example in Meyer plates. (Then again, seeing it in a plate doesn’t necessarily mean Meyer is suggesting that it is the best course of action or that he agrees with Liechtenauer.)
The third interpretation that I can think of is somewhat strained. He could be saying that if you are high the right, then you are at a disadvantage if you angle your blade across to fight someone whose blade is on their own right. But really, that doesn’t sound right to me so for now I’m sticking with the 2nd interpretation.