a) Put your metering to evaluative
b) stick a falshgun on top of your camera (in my case a 580 EX)
c) now put a single color of anything in front of the lens, this is so that you get a constant reading from the reflective meter, maybe a colored paper.
d) put your camera on AV
e) set an fstop maybe lets say f4.
f) turn on your flashgun
f) read the shutter speed chosen by the camera (just set it to 0 for simplicity)
ok now do this
g) turn off your flashgun
after turning off the flashgun the meter will still be at 0 position but the shutter speed will be reduced. Ex, if i had f 4.0 and SS 80 when the flashgun is turned on, the moment i click the power off on the flash it will immediately go to f4 SS 40 but the meter is still at 0.
I know using a flashgun in AV mode defaults the flash to fill in flash. But i thought id keep my settings the same but the electronics will output a lower amount of flash to keep a -1 stop flash fill ratio. I never thought my shutter speed will move and flash isnt affected by shutter speed anyway.
Now do this, move your metering to Spot mode and you wont see the behavior.
If your camera is at manual you also wont see the behavior, it seems to happen only when you are at evaluative metering and AV (maybe TV but i didnt check that)