Watch the Double Thumb Technique online guitar lesson by Andy Irvine from Bass Mechanics: Crucial Groove

Double thumbing is a very cool technique which creates a "dual tone attack" and emulates playing with a pick. The down stroke has the sound of skin (which is warmer), while the up stroke has the click of the fingernail and a brighter sound. Together they create a great percussive feel and unique layered sonic voicing.