Watch the After Sunday online guitar lesson by Adam Levy from 50 Low-Down Rhythms You MUST Know
Ry Cooder is a master of open-G tuning. He mostly uses the tuning to play songs in the key of G - or higher keys, with a capo. Cooder also uses open-G to play in the key of D, so the open strings are the IV chord instead of the I. This lick is a good example of that usage. Try it with your thumb and fingers.