Copy-paste from
June 10, 2001 > In the end of season 1999 Williams designed a new undertray which used the rule which allows holes in the downwards faced bodywork (like the reference plane and stepbottom) when no other bodywork is visible from underneath. They went to see FIA's Technical Deligate Charlie Whiting , showed him the idea and the final product after that, and he gave it the go-ahead-no-problem.
That rule was originally intended to allow bargeboards, which is bodywork ofcourse, seperately from the car's undertray, but because the FIA doesn't want to write down rules like 'we allow bargeboards like this (see drawing)' but deliberately wants to keep them vague and open to their own interpretation, the rule was formulated as above. You'll probably remember this specific rule because that was the one which had to do with the Ferrari bargeboards in '99.
Anyway, Williams designed their undertray in conformation with that rule last year. In developing the 2001-car they designed the whole chassis around that principle more properly, optimising the splitter position, the sidepods-chassis area etc.
'Suddenly' after Imola IIRC Charlie Whiting went to see Williams and said that 'a team' had complained about the Williams undertray and that they, the FIA, would issue a 'clearification of the rule' where they would declare the Williams solution illegal. After 18 months of using it, mind you!
That's what I meant with the FIA restricting Williams. So now Williams have a chassis, splitter, sidepods, so the whole car properly designed around that undertray ... only the undertray is suddenly dismissed. That's a huge setback, because you can't solve much about the basic feautures of the car during the season.