Watch the Chuck It online guitar lesson by Rich Maloof from 50 Acoustic Guitar Licks You MUST Know
When is a guitar not just a guitar? When you're slapping down the rhythm like a drummer. The percussive sound of strings slapping the neck is a hallmark of acoustic playing, and helps lay down the rhythmic feel even in a slow 4/4. You don't have to slap it every time, by the way if you're laying it down solidly, both you and the audience should be able to feel the beat even on those unchucked beats.