[ARCHIVE ANSWER]
I used the dog to obtain probable cause to search a car where there was refusal.
No matter what the dog did, I made him false alert so that I could search the car. I happened to have a drug dog that was one of the best in the nation, so I also used the dog to assist me in finding drugs. If a dog acted distracted or reacted to
irritants, I searched that area more carefully.
Most K-9 officers have trouble distinguishing between certain body movements and don't always correlate the behavior of the dog to distraction or an irritant.
Missing this correlation is what "tricks" the officer.