Watch the Downstroke Workout online guitar lesson by Angus Clark from Hard Rock Survival Guide: Rhythm

This is primarily an endurance exercise, so as mentioned in the performance text you have to focus on relaxation and breathing to keep it up for an extended period of time. Use the sections of the song where you aren't just down stroking to really relax your right hand and wrist. There are a few concepts from the earlier part of the course at play here. Specifically trying to minimize string-scraping noise when changing chords. You may actually find that you get less scraping noise with more pressure against the string while sliding along the string. You have to experiment with your guitar, your settings, your fingers, to figure out what works. There's also your sensitivity to the scraping sound, it may not bother you. But I guarantee you that there is an artist or record producer that you will wind up working with for whom it is a major problem and you will have to know how to control it. I'll also make the obvious statement here that in order to benefit from the course, you should play all the fingerings exactly as I do. Same fingers, same strings, same positions. There is a reason for everything that's happening here. So another concept we're using here is how a 4th can create a very economical fingering or opportunity for a chord hammer-on. And finally, just listen to the sound of the E chord at the 12th fret on the bottom string as opposed to the 7th fret on the fifth string. It is different and you should take some time to recognize what that difference is sonically and what it means to you technically. Don't take any of this for granted.