In article <5c9dn7$ru9@nnrp3.farm.idt.net>, kluzko19@mail.idt.net says...

>Could someone tell me what the Heck it is? I can set up my camera for
>this and I have no idea what it is, what it does, is it usefull (and
>under what circumstances), or what??
>
>Please tell me, my curiosity is killing me!!!!

The flash can be switched to fire anytime the shutter curtains are
fully open for the duration of the flash (which is usually much
shorter than the fastest shutter flash synch. speed). The usual choice
had been to switch the flash just as the shutter fully opened, but
newer cameras often offer the option of having it switch just before
the shutter closes. This has two advantages: during long exposures,
movement blur in the photo has a nicer time relationship with the
sharp flash image; and, when shooting people posing for longish
exposure times, the (surprising) pop of the flash happens at the
end of the held pose, reducing involuntary movement of the subject
(like blinks) during the exposure. The second curtain sync. can
often be left engaged with no ill effect.
Hope This Helps