Pentax ZOOM 105-R Operating Manual page 31

Bulb mode, approx. 4 sec. (ISO 100)
Bulb-Sync mode, approx. 2 sec. [ISO 100)
Bulb mode (
B )
This mode is useful for long exposures required for
shooting such things as fireworks and night scenes. The
shutter remains open as long as the shutter release
button is held down (within the range of 0.5 sec. ~ 60
min.). When using this mode, you should avoid camera
shake by using a tripod and optional cable switch F.
Bulb-Sync mode ( ^
B )
This lets you use the flash in combination with the Bulb
mode. It is useful for taking a well-lit portrait with a night
scene in the background, for example.