This riff uses power chords and a droning, palm muted open string. When you play the open A string in this riff, you’ll want to apply a palm mute with your picking hand to help muffle the sound from the string. To do this you’ll rest the heel of your picking hand lightly on the strings just before the bridge and apply a little pressure as you pick the string. This will create a slightly muffled tone for the note. When you switch to the notes on the D and G strings you’ll want to take away the palm mute to let these notes ring out like normal.