It was called "backwards masking" This was brought about by several christian groups to lay claim that rock music was purely satanic and that many of the groups that are and have been making albums were purposefully brainwashing the youth and leading them into satanism by hidden messages in songs
while many claims have since been debunked and proven false, there have actually been albums that have had backwards masking
Metallica had even admitted that they put a message on one of their latest albums to those that spent their time searching for such messages "To not mess with things which you do not understand"
Subliminal messages have been used for mostly on advertising to a certain degree
but proof that it actually works is hit and miss
That message was a bad joke as was the whole movie