Up until recently I’ve had two Nikon SB-600s in my lighting kit. In my home studio I used them in manual model with dumb radio remote triggers. And in the field I would often use them in TTL with the camera in commander mode.
Recently one of my SB-600s and my triggers were stolen (bastards!)...
When I am using my Nikon D-90 in commander mode, with the built-in flash set to not be used (--) I still get some light from it in the final image.
I created the image below to illustrate what I am seeing. Note that there is no change in the exposure settings or the available light. And there...