I have now witnessed from the same motorway bridge, this spectacle twice.
A convoy of police vehicles travels along the two lane motorway from Seville to Granada. They choose their spot with some degree of sense. today there were six vehicles. two slow everyone down side by side going up quite a steep hill out of Rio Frio and I mean steep. In the meantime the other vehicles get into position up ahead blocking off the hard shoulder and lane one and get cones out. Another vehicle stands at the end of the cones with the guys equipped with a Stinger. they look as though they just need an excuse to use it. A final vehicle hangs right back and sits on the hard shoulder and "warns of hazard ahead" using his blue lights"
then the pull overs/waving through commence. I couldn't see if all "foreign" vehicles were being pulled over as is rumoured. then it is a case of processing vehicles for any irregularities and dishing out on the spot fines. In fairness you see stacks of vehicles carrying loads bigger than the car itself on a roof rack, sitting on the bump stops etc etc so it maybe is an efficient way of weeding out wrongdoers.
When the queue is too long they pack up shop and wave the tailback through. Quite incredible. Imagine that happening in Britain. there would be an outcry.
As a footnote if you are ever given an on the spot fine in Spain politely ask for a receipt, that way the money has a vague chance of getting into the police coffers and not their pockets.

El varche