That article started off with this...
Basic definition: "Dragging the shutter" refers to when you use a very *low sync speed* with your flash.
I didn't read any further. :roll:
Dragging the shutter refers to when you use a lower SHUTTER SPEED.
In a nutshell, every flash photo is actually (at least) two separate exposures. One is from the flash and one is from the ambient light. Usually, they line up and/or one overpowers the other, so that the resulting photo look like one single exposure.
The flash exposure is a result of the lens aperture, the ISO and the power of the flash (essentially, how much light is bouncing back to the lens). But not the shutter speed.
The ambient exposure is a result of the aperture, the ISO and the shutter speed (but not the flash).
So knowing that, you can use the shutter speed to change the ambient exposure, without affecting the flash exposure (just don't exceed the max sync speed).
You can also change the flash exposure by adjusting the power (or FEC) of the flash, which wont affect ambient exposure.
So when shooting in a dark room that does have some ambient lighting, you can use the flash to light up your subject and you can then use the shutter speed to determine how much ambient (background) exposure you want in the photo. If you choose to use a slower shutter speed, to get more ambient exposure...that is 'Dragging the Shutter'.
There are some other things to think about though. If you use a slower shutter speed, that will give you blur if either the camera or subject is moving. The amount of blur will depend on the shutter speed you use, but also how much ambient light is reflecting off of your subject. If it's fairly dark, you will see less blur because the ambient exposure probably won't be very bright in your photo. But if there is more ambient light, then there is more of a chance that the ambient light will case a blurry subject.
The flash exposure is almost always sharp, and it will often overpower the ambient exposure, but not always. So remember that we actually have two exposures, if you drag the shutter with a decent amount of ambient light, those two exposures won't like up and you'll get 'ghosting'. This is where the flashed exposure looks sharp, but the blurred ambient exposure looks ghostly.
This can be used for a neat affect. It can really give you subject a sense of motion...but there's a catch. Normally, the flash fires at the start of the exposure. So if you are shooting with a slower shutter speed, you get the flash exposure first, then the ambient exposure captures movement after the flash exposure. This looks weird because the 'motion' is in front of the subject. To fix this, you can (if your camera/flash allows) set it for second (or rear) curtain sync. In this mode, the flash fires at the end of the exposure....so the ambient 'motion' is captured first, with a sharp flashed subject at the end. So the subject has 'motion lines' behind them, looking more normal.
Lastly, keep in mind that if you use 2nd curtain sync and you are also using TTL (auto) flash metering, you flash will still use a preflash, that that will still be before the exposure. Normally, the preflash and the actual flash are so close together, most people can't tell that it's more than one flash. But with 2nd curtain sync, there is a delay (the shutter speed) between the preflash and the actual flash. This may cause your subject to blink, or more likely, it will cause them to look away, thinking that the photo was already taken.
