This is the flash's duration data from Nikon flash SB-800:
Full M/1 = 1/1050s
M/2 = 1/1100s
M/4 = 1/2700s
...
M/16 = 1/10900s
...
M/64 = 1/32300s
M/128 = 1/41600s
I know, I know, the TS asking about Canon 420EX, and how can high speed sync at 1/2000s can't stop an action.
Like kietgnoel said about high-speed flash, yes, the flash will strobe, so the entire open window travel (from top to bottom, or from bottom to top) of shutter at 1/2000s is 'kena' by flash's illumination. Thus, we can say it is 1/2000s speed.
When we use flash with iTTL, eTTL, Auto or GN mode at sync speed or below, the flash duration will vary from full power to lowest possible power (in my example SB-800, from 1/1050s to 1/41600s). This flash's duration depends on Lens' aperture, Camera's ASA/ISO, Distance from Flash to subject, and flash's GN power (, and whether the subject absorb or reflect the flash's illumination). Same thing if we use flash in Manual mode, we vary the flash's power from M/1 to M/128, thus varying the flash's duration from 1/1050s to 1/41600s. We can say, if we use SB-800 at night (or dark environment) even if you set camera's shutter speed to 1/10s, your shutter speed is varying from 1/1050s to 1/41600s (depends on flash's duration).
Say when the flash is use only 1/16 of its power, we get 1/10900s, which is enough to stop an action (compare to 1/2000s).
Please do correct me (nicely) if I am wrong
.
Regards,
Arto.