It just so happens that I usually like to teach students wanting to develop a fast metal rhythm technique, this main riff to "Creeping Death", since it is so easily broken up into fragments, and your fret-hand doesn't have to do much. This allows you to focus on the picking.
The main thing to focus on when learning this style of picking is to have a very relaxed picking hand. That includes the fingers, hand, wrist and forearm.
Another important element is the very light touch with the picking. When James Hetfield is striking the strings, he isn't hitting them very hard. It is a very light touch. Even though it sounds HUGE and you would think he slamming those strings, in reality, that would make the attack sound very labored and thumpy (for lack of a better word).
I will show Doga how to use speed bursts to slowly build up the riff while maintaining as relaxed a feel as possible. Eventually these bursts become longer and longer until you can play a fast downstroke rhythm for as long as you want effortlessly ala James Hetfield. This method will help you develop the proper sensation to achieve just that!
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