I was standing firm in Longsword focus the other day, with my sword in posta longa. My partner was doing rising cuts to me through my sword. I realized after a few moments that I had a good opportunity to get some practice in.
So while my partner reset I performed cuts to the air, switching between what I call a dritto posta longa (evolved from a fendente) and a riverso posta longa (having evolved from a riverso fendente).
There are moments like these in every class when you are being a practice dummy. You can practice your stand, changing steps, lunges (when your partner is recovering), etc.
Here’s another good addition to any partner exercise. After my partner strikes them I always try to cover their sword and strike them. I don’t go quickly or even particularly earnestly; if I can place my weapon to threaten them I’ve made my point (ha!). It won’t take long for your partner to get the picture: make sure you’re as covered in your recovery as you are in your attack.