I'll try to explain a little something

Pstring talked about the normal powerchords used by most metal bands.

Now I like to add a more dark sound to my powerchords sometimes. If you ever listened to Death metal you know those "powerchords".

Instead of doing (on the 4-5-6th string)

---7
---7
---5

Try

---7
---5
---5

This gives a much darker sound. It probably has a real scientific name or something but I don't know it.