This riff is played using downstrokes. The movement should come from the wrist not the arm and the pick will want to be angled in order to reduce string friction. The main challenge with this type of riff is in keeping the low open note nicely palm muted to get that chunky sound whilst opening out the power chord. Think of the open note as a bass drum and the chord as the snare and try to bring out that difference in dynamic. In the fourth bar, try to bring out the clash by letting the strings ring into each other.