Normally, the flash fires at the beginning of the the exposure time. But if you go into the camera’s menu, you can change the setting so that it fires at the end of the exposure. If a person or object moves through the frame, ambient light on the moving subject will create “smoke”. The flash at the end of the exposure freezes the motion of the subject.
These were 1.5 second exposures at f11. I used a speedlite flash unit that I borrowed (thank you Dave Cooney!) with the flash compensation dialed down and the built in diffuser in place.
Many thanks to Daman Holland and Destiny Gates for being my models, and to Scott Barton for setting up the stage and ambient light for me!!
In the top photo, the flash stopped action as the car entered the frame, and then ambient light captured the drag so that it appears in front of the car. In the bottom photo, the camera captured the drag in ambient light first, then the flash stopped action as the car was leaving the frame.