Flamenco strumming - any time you use the back of the nails of your fingers (not the thumb) to play a note.
Flick out each finger as if you are flicking a bug off your knee. We are trying for individual, percussive, and rhythmic strokes with each finger, rather than just unfurling your fingers in a blur of sound.
We prepare our rasgueados by making a loose fist. The fist allows us prepare each finger to fire. The idea being that your finger is being shot out of the fist, rather than pushing through the strings.
The hand is supported by the thumb which rests on the sixth string. You will find that a bent first thumb-joint will allow for a better angle to attack the strings (especially the 5th string, which is so important to so many chord voicings in flamenco).