Not to be confused with the band, ABBA switches are so named because they reverse outputs such that they fire in A-B-B-A order. So, your redstone devices will essentially fire in one order (A and then B) when activated, and then deactivate in the reverse order (B and then A).
These switches can be extended to include more than just two outputs, and picture below is an ABBA, ABCCBA, and ABCDDCBA switch.
These switches can be vitally important to some designs, because they make it possible to do some fairly unique things, like simulate motion—even make things look alive. Beyond that, it can make your hidden staircases or passageways look that much cooler when opening and closing.
So, come learn how to make one at this Saturday's Minecraft workshop! That's tomorrow, July 7th at 12:01 pm PST (3:01 pm EST).
Join us on our server at mc.wonderhowto.com and don't forget to use TeamSpeak to get the most out of the experience! Our TeamSpeak server is ts.wonderhowto.com.
I'll see you there!