Trigger mechanisms are the key to high speed photography. Timing is obviously critical and no really good images can be obtained until an appropriate trigger device is set up.
There are two basic functions in any trigger mechanism - delay and trigger. In some cases the two functions are provided by separate units, but in other cases they are both incorporated in a single unit. More sophisticated units may also feature a sensitivity control. The Hiviz website is a valuable source of information and suitable products.
The function carried out by a trigger mechanism is normally quite simple - perhaps nothing more than the closure of a pair of electrical contacts. However, the details are dependent on the particular camera in use and the convenient connectors available. Trigger units may be connected via cables or may use radio transmitters and receivers. The sensors used within trigger units may detect sound, use infrared or laser beams, or just rely upon some form of contact detection.
There is typically a delay between a triggering event and the precise moment when a flash unit should be fired. For instance, in cases where a projectile is fired a subject such as an egg or an apple, the sound generated by the launch of the projectile may reach the trigger sensor well before the projectile impacts with the subject. A precise delay must therefore be built in to the trigger mechanism. Quantifying the delay may be be difficult and in most cases requires a degree of experimentation.