Mystery of Gensokyo: How to consistantly do the Infinite Stairs glitch to access Stage 0
And now everyone can do it! PogChamp
Basically, I found this out while I was comparing successful attempts to failed ones. Turns out, successful attempts always paused on that specific step when switching from Reimu to Marisa. Me and ZF probably built up a timing for it without knowing, which is why nobody else has been able to get it.
Each character has a fixed speed on the stairs, meaning they'll always hit the same "points" on the stairs. Probably what's happening is that Y coordinate you put Reimu in is one that Marisa can't stop on normally. However, putting Marisa into that position allows her to completely skip the Y range that would normally take her off the stairs.
This method should work with any staircase that's the same length (4 tiles, or 12 steps) from the top of the screen to where you exit the stairs. Any other staircase would probably require a bit more looking into, since we'd also have to take the distance from where you enter the room into account as well. It's worth mentioning that for these stairs, the tile I switch on is also one tile (3 steps) down from where Reimu enters the screen.