What is the "tennis ball between cylinders" trick used by flight instructors? From this answer, "A tennis ball stuffed in between two cylinders -
a common flight instructor trick to see if students are doing a thorough preflight".

A place between the two cylinders is not the expected place for the tennis ball to be, but how does in general this "trick" work? I tried to Google but only pull out the mentioned answer.
From aircraft flying handbook (page 2-9),


  The pilot should also check for loose or foreign objects inside the cowling, such as bird nests, shop rags, and/or tools.


This tennis ball is a tool for instructor to see if a pilot does a proper walkaround of the airplane. A pilot should be able to detect this strange object inside the cowling easily if he/she does the walkaround properly.