As a student, if you do not have prior knowledge of the instructor's teaching style or his material, try to find some info, maybe even address him or her directly with any meaningful questions, if you believe the answers might help you come to the event with improved attitude. If the host allows for the recording of the event for later viewing, go for it! However, regardless of whether they do or don't allow it, have a notebook and pencil(s) within reach, I cannot stress this enough! Having it within easy reach enables you to jot down any instructions or comments that seem particularly significant and/or interesting, since written notes are easier to revisit and reference than video. Possibly even more important is that you can write down any questions that pop to your mind during the class, so you can ask them later, without interrupting the flow of the lesson.
Next, typically you will find yourself in one of two situations for the duration of the class - either watching or partaking in the activity. I have done both, and here is some advice... If you are "just" watching (for whatever reason - family issues, time of the day, space etc.) make sure to pay attention to how the participating members are doing. As you listen and look at the instruction, try to see who does a good job of it and who doesn't...and then try to analyze and figure out what the differences are and why, maybe also how the performance should be improved. Essentially, you are trying to think from the instructor's perspective here, thus reinforcing the information you are seeking to retain. Trying to explain something to someone else necessarily strengthens your own understanding of the topic as well.
If you are the instructor conducting the session, there are some steps that should be taken in order to ensure the class flows more easily and enhance students' understanding of the concepts and principles taught. First and foremost, picking the right subject to work on can make a big difference. Namely, some things are much easier to cover and explain without a partner than some others. For example, developing physical attributes or polishing one's jab or hook in boxing makes more sense when done solo, than working on clinch techniques and tactics. In BJJ, work from the bottom may suffer less in similar circumstances than top game of takedowns. In armed combatives, such as arnis and eskrima, footwork and striking mechanics will suffer much less from solo presentation than disarming or counter for counter drills. So, with presence/absence of partners in mind, think about whether you will need any additional equipment to aid the teaching. If you believe there are some items that would prove helpful, be sure to notify the students about having it ready as well.
Following that, write down at least a rough outline of the presentation. Even more so than in the real world, fumbling around while trying to think what to do next is plain bad in an online presentation. Besides, when you have a plan, you can spend the first 3-5 minutes of the class giving the overview to the participants, so they may have some context in which to fit your instruction, and it will also lead to more useful and specific questions from them. It is always a very nice touch if the said overview can be sent to the participants either before or during/after the session.
Finally, if in anyway possible, strive to earmark the final 10 or so minutes of the session as the Q&A portion. The kind of questions you get will tell you a lot about the degree of success with your teaching and student's understanding of it. Some of those questions and suggestions might even provide inspiration for future classes, be it in terms of content of manner of presentation. After all, if you care at all about the students taking your lessons, feedback is a must have aspect.
I truly hope this gives some useful insights into the issue of online teaching and learning, so that everybody involved may enjoy the process more. Also, if you have any other advice, I would love to hear it!