Tim's answer is great, so I simply wanted to add a few things. These aren't specifically related to building strength and speed away from the guitar, but should give you some more options and help your R.H. to become more agile.
By keeping your hand still as you execute a rasgueado, you are able to easily play different rhythmic variations of the basic strum. Before outlining these, I should point out that I am a classical guitarist and not a flamenco player, so I only have a few rasgueado techniques in my armoury. A flamenco player would doubtless use many more variations.
Firstly, keeping the R.H. still allows one to alternate down-strums (with either the index finger alone or with three or four fingers) with up-strums with the thumb. The primary benefit from doing this, is that the nail of the thumb is used for the up-strums, rather than using the fleshy side of the index finger, and gives a brighter, more attacking sound. (The "p" in the example below is for pulgar, spanish for thumb. Also, note the downwards arrows are the up-strums, as they move from highest string to lowest - bit confusing!)
If using three or four fingers for the rasgueado, rather than just the index finger, these can follow each in quick succession, as Tim points out in his answer, giving a dramatic, explosive sound. Instead, though, with practice these downward strums can be slightly separated, to give the impression of quick rhythmic repetition of chords. One of my teachers used to play the opening of Rodrigo's Aranjuez Concerto using this technique, instead of simply strumming up and down with the index finger. This gives a lighter, more subtle rhythmic effect:
And these separated a m i strums can then be followed by a down strum with the thumb to create a characteristic triplet rhythm (found in Bolero rhythms, for instance):
Finally, I use a kind of "tremolo-rasgueado", where I continuously strum up and down with all three R.H. fingers alternately (kind of like the way one would "drum" the fingers on a table). I've no idea if this is a legitimate technique, but it works well for me!
Like I said before, I'm no expert on flamenco technique, but find these few additions to the basic "down-flicking" strum work well for me. It would be great if some flamenco players added some information with other answers...