When you stack skills on top of each other like building blocks, as long as the foundation is solid, you can add a new exercise without detailed instruction.
If the rider has achieved the ability to ride ½ pass right and left with relaxation, if the rider can ask for single flying changes of lead at any spot in the arena without stress in either the horse or the rider, then it is only putting together a string of previously mastered exercises when you ask the student execute a shallow canter zig zag.
Instruct the student to pick up collected canter and out of the corner and ½ pass left from the wall to the quarter line, to execute a flying change of lead, and continue to ½ pass right back to the wall. The student then has to make the decision to end the exercise with another flying change of lead at the letter before the corner or to continue on in counter canter and flying change of lead on the short side, based on the behavior and responsiveness of their horse.
Part of the instructor role is to adequately prepare both the rider and horse for success. Make sure that your student has the building blocks in place so that they are able to try new exercises with confidence and success.