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

So far we have been using just downstrokes on some basic powerchords. Adding upstrokes will double the pace. Here's that lovely E powerchord again, this time picked using constant down and upstrokes. We will pick only 1 string per stroke while palm muting (the low E string). This will enable faster playing, however, we also use some open chord stabs again. Use your ear to guide you, and pay attention to the pickstrokes used in the ActionTab to get this right. After you can do this exercise we shall do some barre chords and then look at more musical rhythm variations.

Upstrokes are easier when palm muting just because your hand is closer to the strings allowing you more control and ensuring your pick / picking finger is naturally close to the correct strings.

Although the normal speed audio in the ActionTab is quite fast, don't worry if you can't get up to that speed just yet. Practice will help you to do that. As long as you can play consistently with the slower audio and get the right timing, this is even more important than gaining speed. Your speed will increase the more you practice, but you must build it up from a solid foundation of rhythm and technique first. The normal speed version is to show you what kind of effect you can get from these exercises with some work.