This exercise is from the Core Skills section on Palm Muting.

Now you can chug your way through an E powerchord, let's start switching between chords. Don't worry, we'll keep it simple for now. Here we play between an A powerchord (A5) and a D powerchord (D5). We use the same stabs and rhythm as in the previous exercise, just use 2 different chords, and a lot less distortion.

Notice that the powerchords begin on different strings, so make sure you are striking the correct ones. Also, the D5 is different to the A and E powerchords - make sure you use the right fingers to play the notes. When playing the A5 we use 1 finger to fret across 2 notes (like in the last exercise for E5), just on different strings. If fretting across 2 notes with your 1st finger isn't for you, then just use your 2nd finger to hold the note on the 2nd fret of the G string instead.

The very fact that you are palm muting means that your hand is much tighter to the strings than when open / loose strumming. One advantage of this is that it becomes much easier to change between chords that are on different strings.

When palm muting you are much more in control of the strings. Your pick is closer and your palm controls the amount of string vibration allowed. Omitting strings is easier for your strumming hand, because your pick (or strumming finger if you have no pick) is much closer, and the strumming hand naturally can only move so far from the strings with each stroke. One related advantage to all of this is speed. Most genres of fast metal styles use palm muting and powerchords almost exclusively.