Avoiding and countering Mittelhau : Evasion of Mittelhau uses the concept of Überlauffen, which counters an attack to low openings with an attack to high openings.
This works, because you have the longest reach when your arms are in a horizontal position, meaning your hands, arms and shoulders are all at the same height relative to your body. As you lower your hands, the point of your sword moves along a circle around your shoulder joint, and the lower you get, the more reach you lose.
Provided both fencers have roughly equal bodily dimensions and weaponry lengths, the one who is attacking a high target like the head has vantage of reach over the other who attacks a low target like waist or knee. Here this geometry is used to avoid and counter at the same time.
Avoiding is best done when you move your body out of danger of getting hit, while also getting into a good position to counter your foe with a hit.