In a short while, the instructor is going to demonstrate the lesson but before doing this s/he should make sure that the trainees have understood what has been said during the explanation.
This is easily done by having prepared, on the lesson plan, at least three questions about the explanation just given. It may be that only one or two of these questions are asked depending upon the answers the trainees give but the trainees may, in fact, ask the instructor a question similar to those that are prepared so it's best to have at least three questions written down.
These questions should be asked in the same way and using the same techniques as those given in the classroom teaching sessions i.e. ask the question first and then nominate someone to answer.