from Brazil: Pumping Nylon by Scott Tennant
This is a flamenco technique which is sadly neglected by many acoustic and classical guitarists. A rasguaedo is really more of a percussive effect than a strum. It is done by hitting the strings with the backs of the nails.

Practicing rasgueados develops the extensor muscles, which are the muscles that move the fingers outward, away from the palm. Many players believe that playing scales with considerable speed and accuracy is dependent upon how quickly we can move our fingers out, not in. This would certainly explain why most flamenco guitarists have the ability to play blazingly fast scales.

For now, practice your rasgueados by anchoring your thumb on the fourth string as you play on the first, second and third strings. It is helpful to begin by alternating only two fingers. Play the examples on the next page with the indicated alternation combinations.

Attack the strings from just above

Not like this:

Below there are some rasgueado patterns that include all of the fingers. Some are traditional patterns, and some are a little out of the ordinary.

The letter “c” indicates the little finger (the pinky, chiquito in Spanish). For all the examples except numbers 3 and 6, keep the fingers extended until they have all finished playing.

Examples 8 through 12 involve an exchange between the thumb and the fingers (either all together or individually). This requires a particular motion of the wrist. The wrist should remain as straight as possible while pivoting, as if turning a doorknob. As the thumb plays its upstroke, the fingers follow it into a ready position. As the fingers play the downstroke(s), the thumb follows them into a ready position. In Examples 10 through 12, the thumb returns to a ready position only after i has played.