Backmasking (not to be confused with 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 popularised 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 analogue and digital recordings. The technique has also been used to censor words or phrases for "clean" releases of rap 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. (Full article...)