Flash synchronization

The flash is connected electrically to the camera either by a cable with a standardized coaxial PC (for Prontor/Compur) 3.5 mm (1/8") connector[1] (as defined in ISO 519[2]), or via contacts in an accessory mount (hot shoe) bracket.

Special electronic flash units for focal-plane shutters fire several times as the slit moves across the film.

These sync modes close the contacts a few milliseconds before the shutter is open, to give the flashbulb time to reach peak brightness before exposing the film.

[5] The Friedrich Deckel Synchro-Compur leaf shutter of the Braun Paxette Reflex offered V, X, and M flash synchronization, whereby V (German: "Vorlauf") was used in conjunction with self-timer.

This allows light to be delivered to the entire area of the film or image sensor even though the shutter is never fully open at any moment, similar to FP sync.

An example of improper synch. Either the flash is firing too late or the shutter speed is too fast (shutter moving vertically). Note the different exposure levels.
PC-socket
The photometric output of the GE Synchro-Press No. 11 flashbulb is shown here. Like all "M" bulbs, its peak output was defined as occurring 20 milliseconds after applying electrical current. The No. 11 had a peak luminous flux of 1.8 million lumens. Its rated luminous energy, Q v of 23,000 lumen⋅seconds is the shaded area to the right of the definitional shutter opening point ( 1 / 800 th of a second before the point of peak luminous flux).