BRIAN SAYS… While the three shutter sync options are grouped in the same menu setting, high speed sync is different to the others. First curtain sync was the standard for cameras with a focal plane shutter, such as most film and DSLR and mirrorless cameras. This works for most photos, but if you want to combine a longer ambient light exposure and a burst of flash it can be useful for the flash to fire at the end of the exposure, just before the second curtain closes. EOS cameras include second curtain sync settings for when a Speedlite flash is on the hotshoe.
First curtain sync can make your subject appear to be moving backwards with long exposures