The rasgueado is commonly notated as 3 downward strums that are rhythmically close to each other and ridiculous to play using only downward strums. Sometimes, 'Rasg' is written above the notes.
A rasgueado in fingerstyle is a simpler, shortened version of one of the flamenco rasgueado techniques, and is used to strum multiple times in the same direction in quick succession to put emphasis on a strum. In fingerstyle, Instead of starting the rasgueado on a downbeat, the rasgueado is commonly used to transition from one chord to another, landing it's last strum on the downbeat instead.
To play a rasgueado, prepare by pushing your index, middle, and ring finger into the base of your thumb (medical term is "Thenar Eminence") and flick outwards, one at a time, starting with your ring first, then your middle, and finally your index.
The back of your fingernails should strum across the strings. Once you get used to this motion of flicking each of your fingers out, do it faster with less time in between the strums. Finally, put less emphasis on the middle and ring finger strums and more emphasis on the index by moving your hand downwards while doing the strums.
A softer rasgueado
The rasgueado with the flicked outwards after pushing your fingers into the base of your thumb can sound too aggressive for some applications of the technique. Once you've learned the rasgueado with this method, do the same motion with each finger without first pushing your fingers into the base of your thumb. The rasgueado will sound more gentle and give you more control over each finger.