This Delta-blues style riff uses both minor and major tonalities and keeps the low E string pedaling with every accent of the upper melodic line. The key to making this ditty groove properly is to make sure that this low E string is played just as loudly compared with the upper structure.
The walk-down that is so idiosyncratic in Blues cadences breaks this rhythmic alliance, however, by switching to a somewhat contrapuntal role. And I think that this part at the end of the riff is what makes it so groovy and interesting, so fresh!?
Robert Johnson all over this!