Rasgueo
This is a "flamenco" technique in the sense that it is within that musical context that these techniques are most often found today. In the seventeenth century, Baroque guitarists had and equally developed arsenal of rasgueo techniques, which they called "Battente Style", and the technique itself was called "repicco". In a general sense, it is simply a nifty effect of fingers on strings.
More importantly here, it is a good exercise. It picks up where the single finger strumming leaves off and uses the fingers as a group, in the same way as the single finger practice leads into rest and free stroke combinations.
These techniques even help rest stroke scales, by strengthening the extensor muscles which lift up the finger to return it to its ready position for the next note.
1 - Make a loose fist and rest the thumb on the sixth (lowest note) string.
2 - Shoot out the little finger so that it hits the strings. You can hit all the strings or just a few of them. Keep the other fingers in the loose fist.
3 - Shoot out, one at a time, the ring, middle and index, fingers.
4 - Return to the loose fist and repeat.
The continuous rasgueo is more tricky, and is hard to get without someone showing you and checking what you are doing. Basically, the old style rasgueo is the same as the above, but with no pause after the index finger plays and before the pattern starts again. The only way to do this is to start pulling back the first couple of fingers to their starting position before the last two fingers have played.
The more modern style is to complete the pattern with an index finger up-stroke so that it is a pattern of five strums. This gives the "e" and "a" fingers more time to get back to their starting positions and so is a lot easier. There are many different rasgueo patterns played in many different ways rhythmically. Take some lessons with a real flamenco guitarist (real means: has spent years playing with dancers and singers)
So, the basic patterns to practice are:
1. e a m i <--- fingers
d d d d <--- direction (up or down)
2. e a m i i
d d d d u
3. e i i
d d u
4. a i i
d d u
5. e a m
d d d
6. e a m i p
d d d d u
7. p a i
u d d