Thank you for all the responses so far
Just to give you a quick example of what we use the multitap for in 1 player 2 controller speedruns:
https://youtu.be/VrDkcPhHUWQ?t=43m33s
- Talking to an NPC normally locks your character into place
- But talking with the 3rd controller, then deactivating the multitap with the switch reassigns that character to be AI controlled and I can just walk during the cutscene and mash through the textbox with L/R
Just to make sure you guys understand what I mean: It's not technically unplugging the 3rd controller. Unplugging does not do anything. It's specifically deactivating the multitap-switch on the device that forces this reaction.
What I -think- it does is, it sends the SNES a signal that says, that more than just one controller is going to be connected. Specifically that signal is activated/deactivated when I flip the switch on that device. This prompts the game to check whether there is a 3rd controller active, and disables it if that's the case.
You can actually test this behavior on SNES9x emulator. Set the: "Config -> Controls -> Use Super Multitap (5-player)" in the emulator. Get a 2nd character in the game and press START on the 3rd controller (a small [3] will show up on the characters portrait). Then you just go into the SNES9X menu and set "Config -> Controls -> Use SNES joypad(s)". The 2nd character will instantly be back in AI control. Note, the little [3] in the character portrait will only disappear on screen change