A hot shoe is a mounting point on the top of a camera to attach a flash unit. The hot shoe is shape somewhat like an inverted, squared-off 'U' of metal. The matching adapter on the bottom of the flash unit slides in from the back of the camera and is sometimes secured by a clamping screw on the flash. In the center of the 'U' is a metal contact point. This is used for standard, brand-independent flash synchronization. Normally the metal of the shoe and the metal of the contact are electrically isolated from each other. To fire the flash, these two pieces are shorted together. The flash unit sets up a circuit between shoe and contact - when it is completed by the camera, the flash fires.