Isn't that working just because the whole process of taking the picture occurs within the 1/540 s of the full power flash duration? If so, it is not really high speed sync isn't it?
Correct, but it does equate to faster sync, it isn't HSS, it is what Pocket Wizard call HyperSync, the trick is to get the flash to fire before the second curtain starts its travel, that is what causes the shadow, the second curtain, so if you can adjust your triggering time to sync not when the first curtain is fully open, as in normal sync, but before that, just before the second curtain starts to close and
your flash duration will last the entire exposure at an evenish value then you get faster sync.
But what it should more accurately be called is second curtain sync above true sync speed, but SCSATSS is nowhere near as cool as HSS or HyperSync!