A few notes on speedlites today.
How does a flash set its power level? Not by making itself brighter or darker, but by shining for longer or shorter. Using light in pictures is like using a spray can: the longer you spray, the more you get.
So how long is a flash? That depends. It is 1/1000th second on full power. 1/2000th second on half power, 1/4000th on quarter power, and so on. So if you want very short flashes, fire at low power.
How do I fire at low power? Just get closer to your subject or use higher ISO settings. Then set low power (if you use TTL, the camera will do that automatically for you).
But what if I use high-speed flash? Then rather than firing one flash, it fires many very short flashes, at a rate of around 40 kHz. This simulates continuous light, which you need when using fast shutter speeds (over, say, 1/200th sec), where the shutter does not open all the way at any time.
Product release: Those of you who shoot Nikon can go get SB-600s while they last, and then upgrade to the SB-700, a major improvement, if slightly less powerful than the SB-600. And hopefully it will not overheat too easily, like the SB-900 does.