Flash durations are a very important factor in getting a sharp image when shooting action. When you are using a flash in this respect, you want the flash to be the only light that illuminates the subject. Lets think about this real quick; You set your shutter speed to 1/250th, 1/200th, sometimes even 1/125th when you are utilizing your maximum flash sync speed. Imagine shooting fast moving objects being shot with a shutter speed like that… doesn’t work very well.
Your flashes have a speed of their own, much like the shutter in your camera. This “flash duration” now becomes an integral part of a sharp photo. Take for instance, the Sunpak 555 flash, which has a duration of 1/400th at full power. Pretend we are shooting something… say a bunnyhop in a dimly lit park at night. There is definitly no way to shoot the photo with the ambient light. So you take out your flash to shed some light on the subject (get it?) If you shoot the hop with your flash on full power, you are essentially shooting at 1/400th of a second, which is not optimal with a swiftly manuevering bicycle daredevil. You need to power down the flash in order to get a quicker “shutter” (flash) duration. So we dial down our 555 to 1/2 power, which gives us a duration of 1/1000th of a second. Now this is more appropriate for the shot, but still, 1/1000th of a second is still pretty slow, especially with shots where the rider is hauling ass. So we now dial our flash down to 1/4 power, giving a duration of 1/2000th of a second. This is preferable.
There are still a couple of factors in this equation. As you power down your flashes to get a faster duration, the output of the flash becomes increasingly weaker. You need to get your flash closer to the subject. If you are just shooting with one flash mounted on your camera (which is not very pretty) this means you need to be closer to the subject. If you have your flash (or flashes) on slaves, you will need to move them closer to the subject (but try to keep them out of the shot).
This all becomes very difficult when you are shooting in daylight. Whereas at night we needn’t take into consideration the amount of ambient light, now it becomes a problem. Set your camera to the max sync speed and meter for a reading. Lets say that it says at 1/200th shutter you need to shoot at f/8 for a proper exposure. This setting will give us a nicely exposed, but blurry rider. So you whip out your flashes. The goal here is to make the flashes become the main source of light (overpowering the ambient light) so that when they fire, the subject will be frozen in the frame for just the duration of the flash. So think about it… We want the flashes to overpower the sunlight that is shining on the spot. If our meter is telling us that at the given conditions an f/8 aperture is needed, we will have to close down a couple of stops. Why, you might ask? So that when the shutter opens and the flashes pop, only the flashes are giving us the light we need to expose the subject. This is getting difficult to explain… Pretty much we want to close down to probably f/16 to keep most ambient light out of the photo (specifically off of the rider). We then need to place the flashes so that the exposure we are getting from them gives us a reading of f/16. This will ensure us that when we shoot the photo the only light strong enough to freeze the riders motion in the frame is coming from the flashes. Say you shot the photo at f/16 without the flashes firing. You would barely, if at all, be able to see the rider. This is a good thing, because we now know that when we shoot with the flashes, there will be little to no motion blur on the rider (spinning wheels are tough to freeze completely). But we are stuck having to get the flashes powerful enough to get an f/16 reading but at a low enough power that the flash duration is acceptable for the movement. You can move the flashes closer, but risk getting them in the shot. You can power them up to 1/2 power, which would pose a risk of giving motion blur, but at least you can move the flashes out of the shot. The best thing to do is to have more than one flash in the same spot, pointing the same direction and set at a lower power. The output from two flashes will be much stronger than just one, so you can still have them powered down to 1/4 power, keeping a sharp 1/2000th duration.
Let Ralph Bury and Torey Kish show you what I mean.