Built myself one of these (see www.hiviz.com) to try out some high-speed flash photography. Took all evening to get it to work so there was no time to actually use it (see November 18th/19th for that).
Actually, none of it connects to the camera - there is a lead to connects to the flash but that's not connected in this picture.
The camera shutter is held open for a few seconds at a small-enough aperture and a dark enough room that it does not register any significant image except when the flash fires.