I found this image lost in my archives and thought it would be good to share it. I added some explanation to help digesting it 
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I think it helps to understand how high speed sync flash works.
High speed sync in action
The initial image shows the camera firing at x-sync speed: 1/125 in the case of the EOS 30E film camera that we are using for this experiment. Our point of view is located inside the film chamber, right behind the shutter speed, seeing whatever the film or sensor will see.
The first row shows how the pulsed flash bursts of the high speed sync function illuminates the subject as the curtains fly across the shutter. Notice how the curtain seems “ghosted” as the burst of flash continuously illuminate the subject while the curtains fly across the shutter. Of course, the faster the shutter speed, the smaller is the slit between the first and second curtain and therefore the dimmer that the image becomes. This the reason why HS Flash reduces the effective output of the flash unit.
As a means of comparison, the row below illustrates the same shutter speed but without FP flash. Here one normal flash is taking place, partially illuminating the subject through the slit between the shutter curtains. Appreciate how small the slit becomes when the shutter speed increases. The part covered by the curtain would become a black shadow in the final picture. This is the behaviour that you would see with e.g. studio flashes or wireless systems that do not support FP flash.