Watch the Left Hand Mutes online guitar lesson by Rich Maloof from Guitar 201: Breakthroughs
Let's get back into more options to open up your rhythm playing. One great aproach for adding syncopation — displaced beats and accents that add movement and groove to your playing — is to mute the chords with your fretting hand. It’s also referred to as "damping" or "dampening" the notes, since your left hand creates a damper by not pressing down on the fretboard. Just release the downward pressure and you’ll get that chuck sound. Work on mixing strums and chucks into a rhythm pattern. Once you start getting that feel in your bones, you can get more playful with the mutes.
You can practice this technique with open chords, too — just lay your left hand lightly over the strings to get the mute.