Flash guns are not just for blasting light at a subject when
there isn`t enough.  They can be used for all sorts of effects, 
such as fill in flash and back lighting.

Here,  we  are going to look at slow flash.  Many people  are 
not aware that flash units will synchronise with their cameras at 
all sorts  of shutter speeds,  not just the one that is stated in 
the handbook. The fastest shutter speed for synchronisation is
the one in the book, but ALL speeds slower than that are also OK.

So, it`s possible to set a long speed, use the flash to light
the foreground, and a time exposure for the background.

For example,  set up a model in front of a lighted street at
night. Set the camera on a tripod and focus,  probably with the
aid of  a torch on the main subject.  Attach the flash and work
out the correct exposure for the model.

Beware,  auto exposure will not do. The camera system is likely
to try and light up the background ! Measure the distance from
camera to model - read it off the focusing ring. say it is 5
meters.  Divide  that distance into your flash guide number for
the  film  in use - in meters. If that were 40, the result would
be `8`. That is the aperture to set on the camera lens. A flash
meter can be used outside and would do this for you.

Now,  shutter speed ? We have a lighted street, so possibly 1
second  at f8 ?  There is an element of guesswork here,  but 
several tries at different settings could produce more than one 
satisfactory, but different, result.

So,  set 1 second and take the picture.  The model should stay 
as still as possible, but the sharpness will depend on the speed
supplied by the flash, and that is VERY fast.

Again, use your imagination !