High-speed photography is the process of creating photographs at an extremely short exposure time of less than about 1/5000 s. There is specific techniques. The short exposure time is usually not made dependent on mechanical system only, but by the duration of light with the use of electronic flash units. Even with standard flash units and a good DSLR, approximately 1/5000 s and shorter exposure time can be achieved. This is an informative article on High-speed photography only, we will suggest to read good books to get the method mastered.
High-speed photography : Introduction
Modern electromechanical system can achieve the required speed, fast moving objects, however, by the relatively slow movement of the full shutter mechanism, causes the so-called rolling shutter effect resulting in distorted imaged. Digital image sensors must be able to process such short pulses. Additionally there is the shutter lag.
For very basic High-speed photography setup, what is done is; the photo is taken in a completely dark room. The the aperture is kept widely opened like taking pictures of movements of the stars; the external Flash unit is fired for very short time. In other words, the Flash unit is becoming the controller of the exposure time.
What we actually can not see in our eyes, can be captured in this way.
High-Speed Photography at home setup
But the above trick is too basic and actually it is not possible to bring everything inside a darkroom!
Other than the camera you need one or multiple flash units and a light barrier. A small retro-reflective sensor with pulsed infrared light can be used to control. This photocell is operated with simple AA batteries, a relatively small unit and easy for the amateur photographers as it is affordable. They are available with the connectors for most popular DSLR cameras. It still lacks the flash unit. Studio photographers will argue that they can use their already existing studio flash system. The flash duration of most lightning speed is unfortunately too long, since it is between 1/500 second and 1/1500 second, depending on the model and the set flash output. Thus, for high-speed photography they will not work. The Flash units need not to be powerful, but much faster. The flash duration can sometimes needed below 1/10000 second. One such known company who manufactures such unit is a German company (found from Flicker) :
The reflective sensor consists of a transmitter with integrated receiver and a reflector. These two parts are opposite by function, so that the pilot light rays can be set back from the reflector to the receiver. After connecting the sensor with the camera via a special cable release, it is ready : when something breaks the light beam it transmit the signal and the camera flash units flashes and shot is taken.
Since Joker (that brand named unit from the above link) has a response time of approximately 0.013 seconds and a modern digital SLR has a trip delay between 0.04 s and 0.01 s, a planning is needed. In practice, it is much easier, the light barrier if kept at a suitable distance from the subject and a few test shots are taken, the ideal release time can be determined.
The entire light assembly must be tested before recording by a screen test. One should also take notice of the correct illumination of the background. The more effort you provide, the more attractive the result will be. If you want to shoot drop of liquids or falling objects, protect the moisture-sensitive parts such as camera, photocell and flash equipment. This can be done with a glass plate in front of the camera. If that is not possible, you should at least use a clear glass filter to the lens. Drops of water or even milk are difficult to remove from the front lens.
You are not understanding the Pilot Light thing. I can guess, because I was confused too. Their website has a good explanation about light barrier :