What is the "tennis ball between cylinders" trick used by flight instructors?
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.