Pentax K-5 Operating Manual page 9

Trailing Curtain Sync discharges the flash immediately before the shutter
curtain closes. When shooting moving objects with a slow shutter speed,
Trailing Curtain Sync and Slow-speed Sync produce different effects
depending on when the flash is discharged.
For example, when shooting a moving car with Trailing Curtain Sync,
trailing light is captured while the shutter is open and the flash captures the
car immediately before the shutter is closed. Therefore, the picture will
include a sharp, well-lit car with trailing light behind it.
Slow-speed Sync
(Leading Curtain Sync)
Trailing Curtain Sync
1 Set the mode dial to any mode other than., X or ,I_.
2 Press the four-way controller (T).
The [Flash Mode] screen appears.
Select _ or >_ and press the
OK
button.
Press the _ button.
The built-in flash pops up.
5 Take a picture.
_; Trailing Curtain Sync slows the shutter speed. Use the Shake Reduction
function or turn off the Shake Reduction function and use a tripod to avoid
camera shake.