First of all satanists don't have or use mudras. What you're construing as a mudra is just a hand sign that crazy rockers started throwing up at heavy metal concerts to indicate the horns of the devil and has absolutely nothing to do with meditation or the channeling of energy. The stigma surrounding it is completely arbitrary and cultural.
There is a similar mudra -- the apan mudra -- to the hand sign you're talking about, except that the tips of the fingers are touched together instead of the middle and ring fingers being held down by the thumb. This mudra is good for the circulation, and although I'm not aware of any mudra that is exactly the same of devil's horns hand sign, I'm sure there is one and that it has similar positive effects to the apan mudra. The variations on mudras are nearly endless.
I think Max said that the middle finger to thumb mudra symbolizes bringing down heaven to earth or spirit to earth or spirit into us. That's all.
Incidentally, I've been practicing kunlun with every type of mudra I can find. Haha. The effects are amazing. Here are some links:
Yogic Mudras
The Kuji-in
Always remember to play and have fun. :-)
sadhaka