There are lots of brilliant things you can do in Super Mario Maker 2, using items and enemies in ways the developers likely never even conceived could occur. And of course, these setups are key to ensuring your Mario Maker level is as difficult and evil as possible. So using the Mario tech to force Mario to complete a level only looking backwards is not only possible in Super Mario Maker 2, but it feels like it's in the right spirit too.
YouTuber and Mario creator Ceave Gaming posted a video breaking down the tech, and why it's possible now versus why it wasn't in the first Super Mario Maker. It's a fantastic video, so while I'm going to do a short breakdown below, you should really watch the video to get the full effect.
Essentially, the idea is that Mario has to run over a series of bricks. If he looks forward, that will trigger a switch that turns the bricks into coins, plummeting the plumber into the bottomless chasm below. How exactly can you do this? With the help of a friendly—or not-so-friendly—Boo.
Basically, Boos are a way for Mario Maker to track where you are looking at all times, because they will only move when Mario isn't looking at them. By exploiting this, along with their collision boxes and the physical contact of other enemies, you can create a setup that will detonate a P-switch if Mario looks the other direction.
While that contraption in its most basic form works in a vacuum, and all those elements were present in the previous Super Mario Maker, issues arise when you factor in Mario needing to move across a horizontal plane. Enemies get deloaded and Boos get displaced. Certain things begin to break upon practical application. It's the On-Off Switch that makes the levels actually work this time around.
By placing a Bullet Blaster on top of a Dotted Line block, and an activator further along, you can create a switch that will track the progress of the level and keep the ruleset on. While horizontal levels require building the set over and over across the level, vertical levels only need one. It's a fascinating setup that should produce some awesome, terrifying levels.
If you've been enjoying Super Mario Maker 2, be sure to check out what levels we've been enjoying so far in our Man vs. Mario Maker video. And maybe check out some of the other wild courses, like this upside-down Panga level that seems similarly diabolical to a backwards-only level. You have to wonder how Nintendo feels, seeing people develop such challenges using On-Off Switches and a few shells.