The
technique of singing on an
album in such a way so that when the recording is played
backwards, a different message can be heard. There are two types of backmasked messages: engineered and phonetic. An engineered backmask is created by recording a message normally, and then reversing it.
Pink Floyd, apparently not without a sense of humor, put the following engineered message into "
Empty Spaces":
"Congratulations. You've just discovered the secret message. Please send your answer to old pink, care of the funny farm..."
A phonetic backmask is one in which the creator (supposedly) selects the proper lyrics and alters his speech pattern slightly in order to create a message in reverse. These are the ones that are considered more devious, and often times, Satanic. The most famous phonetic backmask is probably from The Beatles' "Revolution #9", whose lyrics "Number nine", when played backwards, sound a lot like "Turn me on, dead man." Perhaps more proof of the Paul is dead conspiracy. Another famous one is Queen's "Another One Bites The Dust", which becomes "It's fun to smoke marijuana."
After two teens committed suicide, Judas Priest was put on trial for allegedly backmasking messages of suicide into their songs. Christian groups also called for a boycott of Led Zeppelin when they "discovered" that the lyrics "there's still time to change the road your on" from "Stairway To Heaven" became "Here's to my sweet Satan." when played in reverse.
Actor Michael Anderson ("Twin Peaks") can speak in backwards phonetics, and he taught it to some of the actors while filming the show. That's why all the people in Black Lodge speak in a really weird language - they're speaking in backwards phonetics.