Pro 1 Flash Question

Canon's Pro 1 User Manual makes very brief mention of "Slow Synchro" without explaining what it is. Can someone fill me in? Is it simply setting the camera to a slow shutter speed? If so. how slow?

Thanks,
Mark

Slow synchro is a setting where the camera reverts to a slower shutter speed in order to expose the background correctly while still firing the flash to expose your close subject correctly also. If you were to shoot with the flash with slow synchro off, your subject would be illuminated but your background would be dark in comparison.

There are two settings associated with slow synchro, one allows the flash to shoot first and the other allows the flash to shoot at the end of the slower shutter speed