the flash x-sync is the fastest shutter speed for a camera-flash combination which ensures the full image can be evenly lit by the flash without using SuperFP mode (assuming the flash is evenly lighting the scene)
the faster the shutter speed, the lower the ambient light exposure, thus a fast x-sync is very handy
x-sync is dependent upon:
actual travel speed of the shutter curtain - the faster the better
the distance the shutter curtain needs to travel - smaller sensors should have an advantage
synchronisation delays with flash units
remote TTL mode results in slower x-sync - usually 1/200th sec
flash unit technology
older flash units such as Olympus FL-36 and FL-50 will sync at 1/160th sec on most cameras (1/250th on the E-M1)
latest flash units such as Olympus FL-600R may sync at 1/200th-1/320th sec depending upon camera
using shutter speeds faster than x-sync without using Super FP mode
shutter speeds faster than the x-sync will result in progressively larger areas of the image from top down, which are “blacked out” to the flash but still show ambient exposures
Panasonic cameras will not allow you to ever go faster than the x-sync without SuperFP mode
Olympus cameras will allow you to go faster than the x-sync if the attached flash is only connected via the centre flash pin or a PC cord
this is very handy in bright sunlight and one can use 1/400th or even 1/500th sec speeds (even 1/800th sec may be usable on the E-M1)
SuperFP mode
flash fires in pulses to allow faster shutter speeds to be used, and thus wider lens apertures in bright sunlight, but flash output is severely reduced progressively as shutter speed becomes faster