Specifically, this TAS abuses the fact that SMB doesn't clear RAM on bootup to use SMB3 to write $16 to the "continue world" RAM location, and then hotswaps to

Specifically, this TAS abuses the fact that SMB doesn't clear RAM on bootup to use SMB3 to write $16 to the "continue world" RAM location, and then hotswaps to SMB1 to start the game in World N-1 which makes the rest of the TAS possible. If you download the TAS and use it with SMB1 on an emulator, the included base savestate will already have $16 in that RAM location, for convenience. The main HN link for this submission has the full technical writeup.