What is most important to you as an instructor is the ability to understand sail theory well enough to be able to comfortably teach it to your students, and be able to answer most of the questions they might have without having to search too far for the answers.
In their learn-to-sail course, your students were introduced to the basic “push- pull” concept that air flowing over a sail acts to “push” or “pull” a boat through the water. Review this concept with them and make sure they understand it.
The key points to emphasize are:
Push Mode On a run, a sail blocks the wind and causes the wind to push on one side of the sail, and “pushes” the sailboat forward (lift is not generated). To simulate this on land, tell your students to stand with their backs to a strong wind and feel the wind pushing them forward.
If you hold your hand out the window of a moving car with your palm facing the wind, you can feel the wind “push” your hand back. This is how a sailboat moves forward in ‘push mode’ when the wind is coming from behind.