If there were ever a guitar style that ruffles the pickers paradigm, clawhammer is it. This old-time banjo technique fits like a square peg into the round hole that is the average guitarists world view. The emphasis in clawhammer is on down-picking, using the back of the nail, not up-picking. And its not really even what we think of as fingerpicking. The finger moves very little, if at all. Momentum is generated by movement of the whole hand. And the thumb doesnt pluck; it does a push and release maneuver, pushing down on the string and releasing with an abrupt upward motion.
~~ from the liner notes by Steve Baughman