Watch the The Flat 5 online guitar lesson by Jimmy Heffernan from Dobro Handbook
Now we're going to use the thumb syncopation along with the flat 5 and flat 3. You get a greasier sound by introducing the flatted 5th and sliding into the flatted 3rd.
Thumb sycopations help give this lick more rhythm and help give you a fuller sound than just playing the top strings.
This technique is used by every top player from Mike Auldridge to Jerry Douglas to Rob Ickes and so on down the line.