This exercise is from the Core Skills section on the Hammer On / Pull Off Combination.
This is another 'riffing' exercise using the Hammer on / pull off combination. This aggressive style of metal playing involves the Hammer on / Pull off combination. Notice that even the G powerchord involves a hammer on / pull off, which provides a decent rhythmic / melodic effect....Much more so than just playing a standard G powerchord.
This type of riff is all about timing. You must keep the palm muting on the Low E string tight and regular. However, open pick the notes before the hammer/pull offs (i.e. stop palm muting!).
Like the last exercise - use the time that your picking hand is palm muting the low E string to quickly move your fretboard fingers into position so there are no uncomfortable gaps in the rhythm.
This style of metal riff is quite typical of Bands such as Sepultura, Megadeth, Slayer etc. Check out the Angel of Death riff by Slayer for an excellent example.
Again when practicing this riff - apply the golden rule, get it right at a comfortable speed first, then work on speeding it up later!