They rigged the emulator so it would record every attempt and then overlay them and play them at the same time. It’s a modded Super NES emulator that’s rigged to play custom-made Super Mario World levels.
“Basically, there’s a video that’s been floating around on the Internet for awhile. But an interview with designer Tommy Refenes revealed how McClure’s Mario hack came up: McClure, a self-professed fan of “punishment platformers,” played Super Meat Boy without realizing she’d influenced it. Super Meat Boy would enter the picture a few years later. It was an era before streaming was available to the masses. Most kaizo videos on early YouTube and other services showed the “clean” run, when everything goes right, the result of saved states used to inch forward, jump by jump. McClure became fixated on a YouTube comment that ruminated on how the appeal of watching people play kaizo levels isn’t seeing a perfect run, but how long it takes for them to climb the mountain. The term “kaizo” has become pretty popular these days, but in 2008, it was genuinely new. In 2008, McClure became fascinated by kaizo levels, ROM hacks of Super Mario World that attempted to create incredibly tough levels that usually required pixel-perfect timing to solve. IN THE SAME GAME (mario maker) THEY TOOK DOWN MY VIDEO OVER.” “What *does* make me angry, actually really angry, is they copyright-takedown the video showing I had the idea first, and THEN they use the idea. “Now, I'm not angry Nintendo is using an idea similar to one I had first,” said McClure on Twitter earlier this week. It’s not hard to see the leap from McClure to Meat Boy to Mario Maker, and it’s also hard to imagine Mario Maker existing without kaizo hacks.
Part of video game design has been, for better and worse, taking ideas from other games and building on them, legally and illegally.