"I don't expect Ron can much be critisized for thinking the FIA is biased after the FIA let Ferrari measure their own barge boards with their own tools and jigs at the Maylasia DQ appeal hearing." - Dr. Austin
"Firstly the Malaysia 1999 incident, regarding the barge boards, they were 'illegal' , ferrari mangaed with their lawyers to re-write the FIA rules" - The Beer Baron
There used to be some rather good graphics on the net that would help this explanation, but I'll do my best.
The rules state that the bottom of the car must be flat (basically between front and rear tires), and when viewed from the bottom (pretend you tip the car upside down and look at it from that vantage point) nothing can protude beyond the outline of the bottom except the mirrors. If legal external parts are attached, barge boards in this case, they also must conform to the above rule. The obvious purpose of this is so a "foil" affect cannot be created as an aerodynamic aid. There is a tolerance allowed.
Ferrari had bargeboards with a "lip" that extended out from the bottom of the bargeboard, and parallel with the track surface, or on the same flat plane as the bottom of the car. When measured at the circuit, the Ferrari's were deemed to have a portion of the upper bargeboard extending beyond the bottom outline of this lip. Beyond the allowable tolerance.
In the appeal, only the bargboards were present to be measured. Ferrari proved that if tipped slightly, where the lip was not exactly on the same "flat" plane as the bottom of the car (but still within tolerance), the upper portion of the bargeboard was within tolerance of the "footprint" of the lip. Hence the reference in the following article about "how flat is flat?".
Were the bargeboards mounted on the car at that angle? Probably not. But it couldn't be proved differently. Regardless whether Ferrari or anyone else measured them independently.
Was it a good appeal? Yes.
Excerpt from an article on the appeal results.
"The team had been expected to pin their appeal on the argument that the infringement did not affect the cars' performance, but Mosley said Ferrari had convinced the hearing by showing instead that the boards were not illegal.
He said there was a margin of tolerance regarding measurements relating to the flat bottom of the cars -- stating "how flat is flat?" -- and Ferrari had shown they were within that.
"Ferrari came with a very accurate jig and were able to show the court that the turning vane, when at a certain angle to the car, had no dimension which exceeded the tolerance of five millimeters," Mosley said.
"The court of appeal decided to overturn the decision of the stewards and therefore the original result of the race stands in its entirety.
"That is to say, the drivers have the points that they earned in the race and so does the constructor."