As the shutter is released, the metal blades fall down(or go up), exposing the sensor. At the precise moment, a second set of blades falls down(or go up) to block the sensor. At faster shutter speeds, the second set of blades (curtain) closely trails the first such that, in effect, a slit of light moves across the sensor.
Modern Metal Vertical Focal Plane Shutter
This helps explain why digital SLRs have a maximum flash sync speed of about 1/250 second or less. At faster shutter speeds, the blades block off the sensor before the flash is able to fully expose the sensor. In other words, the first curtain opens, fully exposing the sensor. Just before the second curtain descends, the flash must fire while the entire sensor is exposed. Otherwise, a portion of the sensor is covered and produces a dark band across the image.
Courtesy of Photography in Malaysia
Most modern flash units have a mode called High Speed Sync (FP Flash) that allows shutter speeds faster than the camera’s top sync speed of 1/125 to 1/250. It works by pulsing the flash to emit a series of flashes as the small slit travels across the sensor. Less expensive flashes like the Canon 420EX have a set pulse rate while more expensive units like the 580EX can be set to specific pulse rates.