I understand it now in the following way:
either change the aperture (by 1 stop in the example) and leaving shutter speed and flash output unchanged
leave aperture unchanged but change flash power output and shutter speed both by 1 stop
Why do I ask this?
I have had an interesting discussion recently with Kurt Amsler (a well known underwater photographer) and he told me about today's new concepts regarding the virtual no noise sensor cameras at higher ISOs like ISO 400 (I have a D4s now).
Think about the following scenario:
You shoot a dynamic scene (let's say fast moving sharks) in relative deep water with dim ambient lighting conditions. You shoot at f8 and 1/125 sec, 2 flash guns at half power each. The image on the LCD looks ok on the exposure of the shark (foreground) but is too dark on the background, almost black and thus rather underexposed (let's say by -1 stop). But we want blueish water !
Now, instead of lengthening the shutter speed to 1/60sec you can do as alternative: change ISO from 100 to 400 (2 stops) plus lowering the flash output to 1/8 power and thus reducing backscatter and decreasing flash recycle time, leaving the aperture unchanged (relative closed at f8) thus conserving DOF (nobody shoots today with f2.8 - 4.0 anymore!), shorten the shutter speed to 1/250sec and thus freezing the action of the moving subjects even more...
Off course, that works only with a camera which can handle ISO 400 on a neglectable noise level.
Wonderful world, isn't it ?
Anybody else to chime in?