Freeze Motion at Slow Shutter Speed using Flash 

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Green Plasticbag as the flash duration is so extremely short (like 1/20000 of a second) the light from the flash bounces off your subject only for a short time, hence this is the main light hitting the sensor.... it therefore freezes the motion.

+Green Plasticbag Assume you open the shutter for an amount of time and in that period, it records 30 frames and stacks them on top of eachother, out of the 30 frames, your flash synced with 1. That one frame is too bright so the rest don't show.

+Green Plasticbag dude, set the shutter speed to capture as much as u want of the ambient light; in a disco u have to go like real slow, 1/10-1/20; set yo flash to rear curtain, so it will flash at the end of the exposure, so yo sensor can capture the ambient light

While you use low shutter speed in low light situations to gain more light... and when it is not enough you need a flash. That flash compensates for the slow shutter speed, because it makes fast spike of light that draws into your sensor.

Once the shutter opens, it catches whats lighten and freeze it.

+Green Plasticbag Assume you open the shutter for an amount of time and in that period, it records 30 frames and stacks them on top of eachother, out of the 30 frames, your flash synced with 1. That one frame is too bright so the rest don't show.

+Green Plasticbag Because flash duration is so short and fast when it fires, that's how it freezes motion. With a really slow shutter speed, however - like 1/4 sec., 1/13 sec., more ambient light is being let into your photo so the flash will freeze your subject but you'll still get motion blur because of the extremely slow shutter speed letting it a good amount of the ambient light. Remember that your shutter speed controls how much available (ambient) light is in your picture. It doesn't affect flash exposure for the most part. The slower the shutter speed the more ambient light is let into your camera, even when you use flash. Generally, flash fires at a much faster speed than ambient light so it freezes anything that it illuminates. So it would stand to reason that when you use a super slow shutter speed like 1/4 of a second, wherever the ambient light is in your picture there will be motion blur, while the flash freezes your subject anyway because of its fast burst of light. Hope that helps.

Taking the first photo as an example: the exposure time was 1/4 second but the flash fired in a much shorter time - maybe 1/10000 second or something like that. So, you have a relatively long exposure time but an extremely quick flash time that occurs right at the end of the exposure. The guys in the foreground are lit by the flash and they are sharp because in 1/10000 sec they hardly moved any distance at all. The back of the hall is blurred because it is lit not by the flash but by the available light, which was collected over 1/4 second - and in that time, the people at the back moved and the guy holding the camera moved too. This kind of photography is all about having a long enough exposure to let in enough light to expose the background, while using flash to illuminate the foreground. The guys in the foreground would have looked exactly the same no matter whether the shutter speed was 1/4 second or 1/10000 second. It's all about the flash being way faster than them. The only thing that would change with a faster shutter speed would be the wobble in the background would be reduced, but it would also get darker because less light would be getting into the camera.

Green Plasticbag when your shutter is open and your sensor is exposed during the longer shutter speeds the sensor will pick up any movement and blur the image. But, if you have flash enabled the flash comes milliseconds before the shutter closes. This freezes whatever the flash illuminated right before the shutter closed off the light to the sensor. So anything the flash has illuminated will be frozen, but the further away from the flash the more blurry it will get because the light power is significantly reduced. Hope this makes sense.