As discussed previously, an instructor needs to watch the whole evolution. This is for safety as well as the debrief afterwards. If you notice any unsafe activity beginning to take place, warn the student and explain immediately. You can also quiz the students for alternative approaches. Add hypothetical changes during your talk if that helps as well. Our goal is to get everyone thinking, asking questions, and learning how to best use their training.
For some exercises, I like using video and then playing the scene back. In the heat of an exercise, things happen quickly and sometimes without thought. Video is great for capturing subtle mistakes that can easily be overlooked.