Shutter-drag is a technique to capture streaks of light by slowing the shutter while using a flash.It can be a tough technique to master but when you nail one it's incredibly rewarding.
The best way is to identify without the flash which lights you want to see. Prioritize keeping your iso lower so that you can keep your shutter slow enough that when you move the camera you get motion blur.
Next you want to set your flash. A shutter drag is almost like a double exposure and the subject is exposed first with the flash. The flash preserves the moment and then the extended shutter adds streaks to the image using lighting fixtures and other light sources.
The trick is to keep the photos from becoming too obscure. Every time I take a shutter drag I flick my wrist in a single direction or snap the zoom and plan the route so that the lights don't trail through the subjects face. It's not always easy to avoid. Playing around with how you move the camera and how fast you set your shutter is an engaging way to spend the end of an event.
If you are using a zoom lens try zooming in right after you snap the shutter and it creates a tunnel like effect.
