The term refers to the the phenomenon of one's face muscles inexplicably contorting upon hearing an extremely metal riff. It is uncontrolable, many have researched it but it remains a mystery. Why do heavy ass riffs make us do this? we'll never know. Also, those who make and/or play the riffs cannot help it either, and are forced to do the metal face as well.
Put on your best metal face for the picture.
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