This one you'll hear all over the place in one form or another; on the start of Albert Lee's "Country Boy" for example. It doesn't have to sit in a country context, but will give a little punch wherever you need it.
Its kick comes from the tonal contrast between the stopped and open G's or D's (or even A's), and the fact that they overlap and clash a little bit. It can add rhythmic dynamics to an otherwise static chord, with the thumb coming off the main bass beats and becoming part of the treble roll. Later on, we'll have it hop between bass and treble functions.