Using the inverted or upside down devil's horns has 2 meanings. Most often seen at Goth and Industrial shows or in Gothic night clubs, it is used as it resembles Vampire fangs and to say "metal sucks". Both relaying to those around you that you simultaneously support the Goth scene (though most Goths detest the Vampire scene, the gesture is used tongue in cheek) and condemn the metal scene, or those who confuse the two. It began as a backlash against new comers to the Goth scene (or neos) who don't understand that Goth exists not as a compliment to the metal scene, but as it's antithesis.
Wow, man. That scary guy looked at my Manson shirt and gave me the inverted devil's horns like he was irritated. I don't get it.