I know a majority of the time they are not cheating, they just found an edge that there isn’t a rule for yet. Here are a couple examples; the length of the fuel hose, the tank size was specific but the rules never said how much hose could be used to get the fuel to the pump. After a couple races were won because they had enough fuel to make it without pitting and others didn’t NASCAR made a rule of how long the fuel line could be. another one was the number of stages you could run on your oil pump, the more crankcase vaccuumm you pull the more horsepower you can make, the oil pumps in the 80’s used to extend past the back of the engine block. another was the volume of the valve covers and oil pan, similar effect as the extra stages in the oil pump. All of these were legal gray areas that gave the drivers the edge and caused NASCAR to create a rule against it.
I doubt that JJ has any clue what the aerodynamic engineers are tweaking on his car. This is one of the biggest areas left with gray area simply because of how difficult it is to word in the rules.
I actually like some of the creativity some teams take to get the edge, one of my favorite examples was when they were welding washers on to hold up the suspensions to pass inspection knowing that they would break off on the first couple laps at daytona. Once the weld would break and the washers were gone the rear of the car would drop out of the wind greatly reducing the drag, one of the reasons there is an after race inspection as well.