From The Art and Popular Culture Encyclopedia
Backmasking (also known as backward masking) is a recording technique in which a sound or message is recorded backward on to a track that is meant to be played forward. Backmasking is a deliberate process, whereas a message found through phonetic reversal may be unintentional.
Backmasking was popularized by The Beatles, who used backward instrumentation on their 1966 album Revolver. Artists have since used backmasking for artistic, comedic, and satiric effect, on both analog and digital recordings. The technique has also been used to censor words or phrases for "clean" releases of songs.
Backmasking has been a controversial topic in the United States since the 1980s, when allegations from Christian groups of its use for Satanic purposes were made against prominent rock musicians, leading to record-burning protests and proposed anti-backmasking legislation by state and federal governments. Whether backmasking can be used subliminally to affect listeners is in debate by both sides.