All I know is the majority of carts with it had a few specific addresses that triggered the bankswitch when written, toward the top of memory right before the 6502 vectors. I'm unsure how they were implemented electronically.

At least one chip (the FE device for Decathlon) is sniffing the 6502 instruction stream because it triggers on JSR/RTS being executed.

http://blog.kevtris.org/blogfiles/Atari%202600%20Mappers.txt

http://www.classic-games.com/atari2600/bankswitch.html