For instance, suppose an instrument student has just been told to fly a heading for vectors to the approach. If I tap his shoulder and ask what the next two things are that he must accomplish, I would expect him to say something like, "Ahh, set the approach course in the nav radio and identify the station." I wouldn't expect him to say something like, "Check the gas, undercarriage, mixture, and prop."
By repeating the process of tapping and asking the question, the student learns to associate the finger tap with the need to think two steps ahead of the airplane.
Using the finger tap as a stimulus is a lot easier than having to ask the same question over and over. And it's especially useful when the cockpit's busy.
It's best to be discriminant about when and where you tap your student's shoulder. Do it when thinking two steps ahead of the airplane is absolutely essential. Vectors to final, entering the pattern, engine-failure simulations, and other similar events are all appropriate times.
Like most behaviors that are repeated often enough, thinking two steps ahead eventually will become a habit. Within a short time, your students will come to associate thinking two steps ahead with the specific event (entering the pattern, vectors to final, etc.) that initiated the shoulder tap in the first place.