Within the flash there is a high value resistance (about 1M ohms) connected from the high voltage supply to a small capacitor (about 10nF) which is then connected to the primary of the trigger transformer (the other side being connected to ground).
Originally Posted by John Koehrer
As the main high voltage capacitor charges, so does the small trigger capacitor. The sync. cable is connected between ground and the junction of the resistor and capacitor so that when the shutter contacts close, the capacitor is connected across the trigger transformer primary. The energy in this small capacitor discharges into the trigger transformer which causes a high voltage pulse to appear on the transformer's output. This pulse ionises the gas in the flash tube and causes it to start conducting, taking the charge from the main high voltage capacitor. It will continue to conduct until the charge is gone, or in the case of thyristor flashes, until the thyrystor circuit stops it conducting.
So the voltage present on the sync. connection is likely to be the same as the internal high voltage supply unless it is limited by a zener diode or resistor network.