I love having the opportunity to step into the classes of other HEMA instructors as well as classes offered by my own students. It can be a tremendous opportunity to be opened to new skills and new teaching approaches. However it can sometimes be a tricky thing, especially if you're a more senior practitioner. Here are a few ideas I've found useful to help approach these encounters gracefully.
Listen and Participate
One of the core skills that I seek to teach my students is how to be an excellent student. Excellent students seek to be fully present in the classes they take. They ask questions that bring them clarity, they position themselves to take in the most visual information, and they follow the prescribed drills to their utmost before innovating. They seek to pull as much good stuff out of the instructor as they can while being excellent partners for everyone else who is participating.
Do that. When you're taking someone else's class endeavor to be the ideal "student" role model.
Hold Your Challenging Questions for Later
Not all exercises make sense at first. Unless you think that what is being communicated is dangerous, hold your questions of efficacy to the end or to a moment where you can pull the instructor aside. Even if you have the best of intentions, having one instructor questioning the work of another in the middle of the class can often derail things, create unnecessary tension, and often puts an instructor in a position where they may find it difficult to answer clearly (if they feel they're having to defend their authority while also answering a question).
If I'm going to ask a challenging question in a group setting, I'm always certain to check my intentions first to ensure they're going to benefit the group as a whole, teacher included.
Honour Thy Teacher
Along with being incredibly rewarding, teaching is also challenging, emotionally risky, and requires a lot of preparation and effort. When someone puts themselves out in front of a group and deigns to teach a thing it's important to recognize the efforts they have put in, mostly for the benefit of others. Thank and acknowledge the work that your teacher has done to make it possible for the class to learn.
Only Give Feedback on Teaching if Invited
Though you may have a lot to offer another instructor, be aware that unsolicited help is often not helpful but is instead harmful. At best it may fall on deaf ears and at worst you can create a bias against what you're offering. I find it's best to give feedback on teaching when that is setup and agreed upon in advance "Will you watch my class and give me some feedback?" or at the very least prompted by the one receiving the feedback.
Affirm and See
We're all looking to be affirmed and seen, especially by our peers and those we respect. One of the most meaningful things I can offer a fellow instructor is recognition of the value they put into their class, what stood out as special, and what they uniquely have to offer as an instructor. If I am truly seeking to honour and improve another instructor, this is truly the place to start, and often end, my interaction around their class.
I recommend creating occasion in your life to teach, take others' classes, and invite those you respect into your own classes to give you feedback and help you improve. Teachers need a positive community as much as students. I'm glad to have you in it.