In some ways like the 917 before it, the 956 took advantage of rules in a way the creators of Group C didn't anticipate.
Could you elaborate, for a non race enthusiast (but curious) audience?
well, i'm not the original responder, but when it comes to racing history pedantry, i'm always game!
first, lets start with the 917. now, if you've seen 'ford vs. ferrari' you kinda have seen the first half of the story. the other half was that the FIA (racing sporting federation) tweaked the regs a bit to curb ford's dominance. they reverted to the GT40 Mk. I and still won two more times ('68-'69). the '69 race at lemans featured a literal duel to the finish between a GT40 and porsche 908 (nominal predecessor of the 917) that featured three lead changes on the very last lap.
by that time, porsche had decided to move up from the smaller classes that it was dominating to take on the big boys in the prototype category. now, remember that rule tweak i mentioned? the rule specified that to homologate a car for the 'group 6' specification, it would need 25 examples built. the underlying idea here was that it would return the class to the type of semi-road legal cars that had dominated endurance racing in the early '60 (think of cars like the ferrari 250 GTO) rather than the very-low number prototypes like the ford GT40 Mk. IV. porsche pretty much just turned that whole concept right on it's ear by building 25 917s before it even raced (and were completely proud of showing them all off - google something like 'porsche 917 homologation' and you'll likely find the photo of them all lined up. ferarri, by contrast was prone to use shenanigans to convince the commissioners that they had created X number of examples of a certain model.)
long story short, 917 dominates european endurance racing utterly in 1970-71, ferrari pretty much gets crushed and leave endurance racing altogether (they had utterly dominated lemans from the late 50's to mid 60s) and haven't really been back since (just because there was a dallara with a ferrari engine that raced a few times in '90s doesn't make it a factory effort.) the FIA does what the FIA usually does in the face of porsche screwing with them and alter the regs to pretty much outlaw the 917. porsche hangs a pair of turbos on an open-cockpit 917, eventually resulting in the 917/30 that raced with something like 1300hp in the CanAm series here in the US - until that, too, got outlawed. exit the 917.
interlude: the forgotten hero in all this is the porsche racers from the mid-70s. it all started with the 934, which was basically turbo charged 911 made for racing in the group 4 category. porsche engineers figured that bigger tires would help them significantly and so the 934 got the most massive fender flares perhaps ever seen. but hey, if we can do that with a quasi-production model, why not try it with a semi-prototype? and thus the 935 was created for group 5. the 935 may well be the most numerically abundant prototype ever created and most likely has won the most races world wide of any car in the post-war era. people were still racing them competitively in hillclimbs and whatnot well into the '90s, so lets not dwell too much on that - except for one thing: the original 935 had the headlights mounted int he fenders same as any other 911, really. until some enterprising porsche engineers realized that they didn't have to have them configured like that. so the headlights went in the front valance and the 'flachbau' porsche was with it's sloped nose was born. if you see a picture of a 935, that's usually what it looks like. so, with group 4 and 5 more or less cornered (porsche was in the business of selling racecars - and boy, business was good back then.) a few enterprising minds turned their eyes to group 6 once more. using a leftover tubeframe from the 917, the (ridiculously powerful for it's era) braking system from the 917/30 and the engine from the 934/935, the 936 was born and yeah, lets just say wins at lemans an elsewhere soon followed.
the common thread here is the 911-derived turbo engine that porsche used in all three models and subsequently developed further for their initial foray into indy (which was aborted because USAC - perhaps rightfully - feared the specter of porsche dominance so much, they basically tweaked the regs to ban the porsche engine before it ever raced) and ultimately was developed even further for use in the 956 and then the 962.
now, to wrap things up - what made the 956/962 another one of those cars that kinda broke all the rules? well, group C, which finally replaced group 6 in the early '80s was essentially a fuel consumption formula. cars under it's aegis were supposed to be limited to no more 100l fuel tanks and no more than 5 fuel stops per 1000km raced. the idea was to curb the explosion of power that the sport had seen in the waning days of group 5/6 (where some 935s were racing with north of 700-800hp.) and in theory, this was a good idea, really. i mean, ford built a group C challenger (the C100) at this same time using the venerable Ford Cosworth DFV formula 1 engine as it's base and it (imo) kinda-sorta was the type of car the FIA had in mind. lancia f.e. exploited a small loophole in the 1982 regs that let non-group C cars race alongside the group C cars and thus had pretty decent success versus the frugal 956 because they were racing with just as much horsepower that they could squeeze out of their engines, not having to worry about the fuel consumption rules. when they switched to their proper group C challenger (the LC2), their successes greatly diminished. so what made the 956 work the regs in such a way as to trounce all comers? well, the article points it out - the engine management system gave them completely unparalleled fuel efficiency while at the same time allowing them to have more horsepower than their direct competitors. on top of that, the engine was a very mature design and pretty reliable (something that couldn't be said for the ferrari-sourced engine in the LC2, though it would later be used to great effect in both the 288 GTO and F40.) the real hammer blow, though, was that they sold it pretty much anyone who was willing to pony up 230,000 deutschmarks (they did not do this with the 936 - only four were ever made.) this pretty much insulated them from FIA tomfoolery because if they did, they would overnight lose pretty much 90% of the field in any given race.
the 956/62 was really the last of it's kind in that regard. sura, companies like dallara supply cars to the entire field in a great number of series, but that's due to the series effective adopting a spec-chassis/one-make only rule. the 956/962 was simply an effective option for thise who wanted it and porsche was more than happy to sell them in great numbers (which, it has to be said, they did not do for either the GT-1 or the 919 in subsequent decades.) eventually, some people did rise to the challenge of taking on the porsche racing hegemony (first jaguar and then mercedes-benz after it more or less was shamed back into racing by peter sauber building a significant challenger using an off-the-shelf MB engine) the 956/962 thus passed into history.
btw, there are literally dozens of really nice liveries for the 956/962 well worth the google.