I'm trying to learn the rasgueado strumming technique, but I just can't get my fingers to "snap" out with enough power to get an effective sound; is this just something that comes with practice or are there strength exercises I can do away from the guitar that will help?
Background: I'm not a flamenco player; mainly folk / blues. I just want to bring in some different sounds for my own enjoyment.
Use a very tight fist, and flick each finger out, most seem to start with pinky. Do it slowly and controlled at first, making sure to hit all the strings with equal force. Do not move your arm/wrist/hand until all four fingernails have 'rasgued'.
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...
After practising at the weekend I'm going to add another answer myself that I found useful :o)
Practice on the guitar, but with muted strings rather than playing a chord; it helped me focus on one thing - the speed, power and consistency of the right hand - without the distraction of worrying about whether the notes sounded right.
It also made it much clearer when I was doing it fast enough - getting a quick, clean "CHUNK" sound with each finger rather a slower "washboard" effect that ran into each other (which I'm sure would be a lovely alternative sound, but is not what I'm after right now).
In flamenco guitar, a technique often used to get a rasgueado to "snap" is to place the upper side of the nails of the fingers used in the rasgueado (mi-ami-xami, etc.) right beneath the middle joint of the thumb. It is similar to the "closed fist" technique described above, except you do not close the fist, rather you form a circle. If you do this even without the guitar, you will notice that you can build up the energy by pressing your fingers on the thumb, and then simply release. Rest your thumb on the sixth string of the guitar and use this technique.
However, this only works if it is a single rasgueado and not a loop of consecutive rasgueados (say, just ami instead of amiami ad infinitum). Of course, rasgueado loops aren't used as accentuations and as such don't require to have a "snappy" sound. The single rasgueados are used for accentuations and require the "snappy" sound much more. For rasgueado loops, the main focus shouldn't be power as much as consistency and making sure that each finger plays all the string separately (i.e., in the case of a ami rasgueado, you play all of the strings with a before the m kicks in(because of the sympathetic muscles, this is hard to do) and the finger movement is quick (no "sliding" across, it should sound like a chord)).
