I must admit I am fascinated by this. Mr McNish says it worked – then it worked.
Having risen to the mighty academic heights of CSE grade 2 physics, I am more than aware that I am way out of my depth here (but since when has that stopped me.)
I would always have imagined that once the car got sideways then the ‘fin’ as a flat surface facing oncoming air moving (relatively) at (say) 150mph (?) would create drag at the top of the car (over the fin) which in turn would cause (opposite reaction) lift at the bottom.
I wondered if in fact this was evident with the McNish accident where the car gets sideways and skips (leading edge up) across the gravel??
As has already been discussed – they worked – McNish said so, and no one was hurt. I guess in the case of his incident had the fin not been there then it might have spun dug in and rolled?? We (and certainly not I) will ever really know perhaps?