Recently FStopper wrote an article about high speed sync and the difference between it and HyperSync.
Pocket wizard also written a more technical write up here:
High speed sync is really one of the best thing that happen for flash photography. However the biggest issue is massive lost of power which from a speedlite results in a output that is insufficient for many purpose. Profoto B1 and B2 provide HSS with a much more efficient output approximately to just one stop lost of light. This is significant since they output much more light then any speedlite.
So before High-Speed Sync was available, I utilised the Rx-1 to get sync of up to 1/1600 using the inbuilt leaf shutter.
Both of the above shot is done in the morning around 11pm. Using the leaf shutter, you can not only kill ambient light, but also synchronise with a flash up to 1/1600. However do note that the speed of your sync also depends on factors such as the wireless trigger (I use a phottix Atlas) and the duration of the flash light.
The issue with leaf shutter: There is really not much cameras and lenses with it, limiting your choices. Also sync speed will reach a cap at around 1/1600. This limits how much ambient light you can kill. However the advantage is that you are utilising the full power of the light. Compared to HSS at 1/1600, you save more then 2 stop of flash power, allowing you to do some multi-shots that can best capture action moments.
Of course all this changed when Profoto introduced HSS and I love it.
HSS allows you to use any lens, any shutter speed, to capture things in the way it would have not been possible without using extensive ND filters. ND filters causes a lot of problems: Darkening of view finder, focus issues, colour tints. HSS allows you to bypass the restriction of the shutter sync and with any lens capture the image. Why do we want to capture something using HSS? The ability to use large aperture lens for the shot resulting in shallow depth of field while properly filling in the shadow with the flash at any shutter speed!
The issue is HSS results in lost of usable light. If you want to maximise your flash, a ND filter is still a more efficient solution. However if you just want to fill against daylight ambient but do not have a leaf shutter system, this is your best bet!
For those who are interested what the Profotos could deliver: B1 in HSS is approximately the power of 8-10 SB910 in gang mode while B2 is half that amount. This allows you to use mods to further enhance the light. However for the amount of power they can produce in HSS, there are restrictions.
Min Power: 7/10 on canon system
Min Power: 8/10 on Nikon system for B1 and 7/10 for B2
At times this is just overwhelming!