Actually, with FP flash as with any FP shutter at high shutter speed, each part of the image is exposed only for the stated time like 1/4000s, but with different parts exposed at different times over a total period of about 1/300s that the shutter curtains take to complete their trip. So motion blur is controlled, but images can be spatially distorted: the famous "leaning over car" effect. Think of FP flash as continuous lighting, but only continuing for about 1/300s.
We just finished shooting a large lifestyle project on two sets. All of the imagery had interaction between the talent, some fast like running, some slower like jumping up on sofas and ended up using the 1ds3' and profotos.
Of the two sets one was quite large as the scenes involved multiple talent, usually 4 models, so the lights had to be a distance away.so I thought I'd probably have to go to specialty lights or bitubes to hold the detail of the movement, but on the more severe movement we just upped the iso a stop or two, cut the power down on the profotos and the imagery was sharp, like full length subjects where the eyelashes were sharp.
I don't know what the flash duration was, but of course the Canons sync is 1/ 250th, though all I cared about was getting it sharp and it was sharp, all handheld, all at about F11.
For real intense movement I use photogenic monolights. They're not sexy, but I have about 30 of them of they've worked for years. If you up the iso to about 640 to 800 or so and cut the power of the monoheads to 1/16th you get a flash duration of 1/4000th of a second and we've used these to hold detail that is amazing, but you do need a camera that can go to clean higher iso.
You also get a recycle time of less than a tenth of a second so it gives you the chance to shoot multiple frames.
With advertising, the rule of the last few years is interaction and freezing the moment. It seems like every creative brief I see mentions reality, interaction, even if the shoot is in studio so you need equipment that is fast to set up and fast to make changes, both cameras and lights.
The way budgets and schedules now run, you don't have the time luxury of taking 20 minutes between sessions to change and fine tune light. It now needs to be a 3 or 4 minute process and we put the lights on stands with rollers so we can make the adjustments quickly.
Now that we add video to most sessions, we do the same with the 5d2. The sticks are on rollers with a few marks on the floor as to where we will set up. We adjust the lights, shoot the stills, roll the video camera in to positon 1, shoot a sequence, roll it in to postion 2 shoot a sequence and on to the next session.
If this sounds like less quality that's not the objective. Client requests now are non compromising. It's everything, the lighting, the look, the posing the styling, but everything has to be faster.
BC.
