I was thinking about it in this thread (post #2).
I would be grateful if anybody can give some contribution.
I have no high-speed-synch flash but I suspect the following:
An M, F or FP synchronization will fire - if I get it right - the flash bulb slightly before the opening of the shutter, to allow the bulb to reach its full luminosity.
An X synchronization will fire the flash when the first curtain ends its trip.
A "rear curtain" X synchronization will fire the flash a little before the second curtain begins its trip.
A HSS synchronization will fire the flash before the first curtain begins its trip.
I suspect that using an HSS flash with an X synchronization (be it on the first or the second curtain) will lead to a partial exposure of the film if a shutter speed faster than max X synchronization is employed. When the flash is fired, the second curtain is already on its way.
An FP synchronization might work with an HSS flash because they both work by firing the flash before the first curtain opens.
It might be necessary to cut communication (excluding hot contact) between camera and flash in case a dedicated flash sets the camera at max X synchro speed automatically as soon as it reaches full charge.
If anybody has a HSS flash and would like to experiment with an old camera with an M or FP synchronization, that would be VERY useful for the community.