Description
Tremolo picking is a technique that is occasionally used by a few players for an effect. Technically, this technique uses specific rhythms that fit evenly with each beat. However, many metal players define this as "picking as fast as you can while fingering notes". This reckless abandon works well with the metal attitude.
When trying this remember to relax your pick hand and maintain a firm grip on the pick. Keep the pick going but focus on the phrase you wish to create with the fretboard hand. This is how to create the wild, reckless sound. Don't worry if you don't have an even amount of notes played per beat. You'll see that I don't, either, and the phrase has a crazy, metal sound.
Here's a lick using both tremolo picking and the Phrygian Natural 3 scale.