As the 1980s turned into the 1990s, mountain biking was a wave still building, and the crest of that growing wave was shaped by cross-country racing. At the sharp end of the 'pedal until it all turns red' set, there was much to be desired in terms of the actual pedals being used. Road racers had clipless pedals, but mountain biking was still somewhere far behind that evolution. Hiking boots and beartrap pedals had given way to Suntour XC-Pros or similar, with doubled-up steel toeclips and two Alfredo Binda straps per pedal cinching down on a Rivat or Sidi cyclocross shoe and requisite cleat. This was not an ideal way to go off-road. The alternative–going full roadie and taking smooth plastic soles and massive triangular Look cleats into the dirt–was far worse. It was fine while pedaling, but any off-bike excursions became an absolute shitshow. It didn't stop people from trying though, since the pursuit of speed was paramount (You kids in the back, in the POC helmets and the Five Tens! Stop laughing!).
In 1990, Shimano dropped a dense little bomb on the scene and promptly salvaged dignity for all. The SPD M737 pedal, an addition to the Deore XT line that year, suddenly brought full clip-in pedaling to the masses. And it did so without compromise. The cleat was small and unobtrusive, and was easily contained within the tread of a shoe that was otherwise totally normal looking. No more walking like a duck and fearing smooth tile floors. The pedal itself was a hefty little brute, but featured spring-loaded retainers both fore and aft, so the rider could mash down and step straight into the pedal, then twist to release. The spring tension on each side of the pedal could even be adjusted. The 'two slots drilled in the sole' pattern Shimano used to mount the cleats became the default drilling pattern for every mountain bike shoe that has come since, and the pedals themselves became so immediately ubiquitous that the word 'spud' was for a while the common term for any off-road clipless pedal. They were also ridiculously durable, as evidenced by how many pairs of the damn things you can still see weighing down bikes today, 27 years later.
As with all new things, there was a learning curve, and a pound of flesh to be extracted. Everyone has a story of the first time they came to a stop still clipped in with their brand-new SPDs. Mine happened at the intersection of Oak and Cole streets in San Francisco at rush hour, when my frantic reflexes took over and instead of simply rotating my heel outward, I tugged frantically upward in a trackstand until slapping over sideways in front of three lanes of impatient traffic right as the light turned green. Some theorists, meanwhile, argue that the art of bunny-hopping may never fully recover from the ugliness that ensued when people began to realize they could get a bike into the air by pulling all their meat upward at once.