Watch the Double It Up online guitar lesson by Freekbass from 50 Freekbass Licks You MUST Know
This lick is all based around your right hand thumb. What I am doing is a root/octave run going between G-C-Bb-F . With four 16th notes on the root, and four 16th notes on the octave note.
With my thumb on my right hand I am doing both down and upstrokes. The thing to strive for when playing this lick is to make all notes as even sounding as possible, so the listener will not be able to tell the difference between an upstroke or a downstroke. It is almost like you are using your thumb as a pick.
Make sure you keep your thumb steady, not wiggling it to create the rhythm.