Watch the Thumbthing 1 online guitar lesson by Rich Maloof from Guitar 201: Breakthroughs
This basic “claw” technique is based on using the thumb to catch the bass notes and pulling on upper strings with the remaining fingers. You might want to start with the thumb plus your first three fingers and then bring in the pinky in if you're comfortable. Just make sure you’re plucking notes appropriate to the chord you want to hear. Play in steady time and you’ll find that your hand naturally falls to the strings with a little slap that’s right in time, too. In the next clip we'll put this technique to work on a progression.