Watch the Big White online guitar lesson by TrueFire from Play Blues Guitar 7: Fingerstyle Approaches
Big White - Lick #41 is a video guitar lesson presented by Joe Dalton and is sourced from 50 Acoustic Blues Licks You MUST Know.
The drop D bass is thrown in between the other notes in the lick. The feel is that there is something constant going on, but we don't need to continually thump the bass. The rest of the groove is just the 5, 6, b7, but the constant D above the moving notes gives us a different feel.