If this video confuses/bores you: Read this post then jump to 10:19.
In warpless SMB3 runs, there's an early (second world) instance of randomness where a hammer brother can run either right or left, and if it runs left you can skip three entire levels and save an entire minute:
https://smb3.bf0.org/smb3mechanics/2018-08-22-early-hammer/
Getting this randomness is important, so players use a "manipulation"– they play in a very precise way (in Mitch's case, literally miming along to a video of a TAS) to get exact random numbers.