The one disadvantage of high speed sync is that it pulses the flash many times over the shutter cycle which will consume some of the battery life, and often provides a bit less light contributing to the overall exposure. HSS, on the other hand has it's advantages with it's use in freezing action.

With flash photography you have two sources of light that influences the overall exposure and the shutter speed controls the ambient light part of it. So, if you want a darker ambient, or you want your flash to freeze action you will need to use a faster shutter speed which will kill more of the ambient light. It's often misunderstood that flash will always freeze action, but that's not true. In trying to do that while use of a slower shutter speed can cause what's known as ambient blur. That happens because the flash fires a very quick burst when the shutter curtains are full open, but with slower shutter speeds, say 1/40 second, the shutter is still allowing light to come in after the flash has fired, and that's what can cause the blur. A while back I was explaining this to someone and I created a little video to demonstrate it, you can see it here if you wish: https://flic.kr/p/2neCGQB

Unfortunately theres no short explanation of the difference. I can make it simple, but not short :-(

High Speed Synch is not truly high speed (aka "strobe") flash. Its not actually flash synch at all. Its not synch and is not even flash. It was introduced decades ago by Olympus who called it by a more honest term, "FP Synch" (FP=focal plane). One could reasonably view HSS as no different than an old time flash bulb ... except its reusable instead of disposable.

Electronic flash has an extremely short duration. HSS lighting is long duration. Its actually continuous light and is NOT flash.

The important difference is whats sometimes called "synchro sun" flash. HSS does not allow for synchro sun.

With continuous light, whatever you do to adjust exposure affects both the ambient light and the additional light equally, thus you cannot alterr the balance between the two sources by adjusting the camera. The only method available is changing distance or adding physical attenuators to the HSS light.

With an actual high speed flash you can control the flash power via the flash controls and separately control the ambient light exposure via the camera controls. Thus you can directly control the flash-to-ambient lighting balance.

Whether using synchro sun or HSS, aperture affects the exposure of both sources equally. HOWEVER !!! high speed electronic flash duration is so short that its unaffected by the shutter speed while acoarst the shutter speed ALWAYS affects the ambient light exposure. THAT (!) is the whole key to using synchro sun for controlling the ambient-to-flash exposure balance.

The key point is that HSS is NOT actually flash. Its a CONTINUOUS source exactly the same as an LED video panel, except it uses a xenon gas tube instead of LEDs.

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If the above makes no sense to you, then just dont buy a Sony a1. That saves you 3 to 4 thousand $$ which you can spend on exotic lenses that will make you the envy of your entire camera club :-)

HSS greatly reduces the duration (thus power output) of your strobe's power to enable pulsing the flash at a very fast rate, so you DO NOT have as much light illuminating your scene as with a single full power flash. .

The A1 with its 1/400 sec flash sync allows you to use the full power of your flash, better to light dark subjects, and better to light larger areas and subjects at greater distances , and better to overpower the sun. This is a huge advantage .

FYI, I am a longtime pro and I currently own Sony A1, A7RIV, A7SIII. And yes I have done all types of flash photos in my 48+ year pro career using all types of flashes and all brands of camera and flashes. This is truly a breakthrough for digital cameras.

Cheers and best to you all.