I played with Eric in the 60's before he went down to north carolina and discovered a kid named Amos Garrett. OK--here's how he did it--you are in the key of G minor. Wrap your thumb around and hold down G on 6th string 3rd fet. then bar the high E and B string on the first fret. That gives you a C and an F note. Do a constant walking bass from the G note to the open D with your thumb. Leaving your index bar on the two strings, hammer onto the third fret on both of those strings. For a sensual variation, hammer on both at once. Also hammer on the G string open to 3rd fret. So you are walking the G to D bass while hammering on and pulling off the third fret on the top three strings while holding your left thumb and index in place.