Generally, you wouldn't use flash to photography fireworks. But when you have someone in the foreground, then it becomes useful to have your subject lit up with flash, to balance them with the background (the fireworks display.)
Photographing people with fireworks in the background, is just an application of the technique known as dragging the shutter. I had the couple in an area where there wasn't much ambient light, so that I could light them mostly with flash. The strobe was a Quantum T2 with an umbrella, used in manual.
My flash exposure Read more inside...
