As you probably saw in the last part of the previous lesson's video, 16th note triplets are a great way to get a machine-gun like effect with your guitar. And that's exactly what we want since we're focusing on metal playing. The riff in this lesson might be comparable to the mid-section of Metallica's "One", where the guitars and drums sync up to play blistering 16th note triplets.
Now before we go further, you might notice that some of the riffs in this tutorial are pretty fast. Don't let that intimidate you though. Just play them at speeds that are comfortable for you, then gradually work your way up when you're ready.
In this riff, we're basically going to play the 16th note triplets every other beat. If you look at the tab below, you can see that the 16th note triplets are played on the 1st and 3rd beats of the measure, while the 2nd and 4th beats have rests. You'll also notice that I threw in some power chords on the 4th beats in a couple measures.
Last thing I want to emphasize, is my left hand. Observe my left hand in the video. Notice how it falls on the strings to mute them immediately when a rest occurs? Be sure to practice that. In order to get that tight metal rhythm playing, your left hand needs to be involved.