This video teaches you how to do the binding parry against Zornhau, and also why it is useful to step back when doing any binding parry.

Binding Parries are for dealing with the likely crossing of swords. The goal is to catch your opponent's blade and get into a good position to counter-attack. Although passive displacement is not really what you should do in a fight, it is not always possible to avoid it. So these exercises are meant to teach you to bind/cross swords to your vantage.