I noticed something strange today and I wondered if anyone else had noticed the same.
Camera: D7000, Flash: SB800, PW mini and AC3 on camera, PW Flex under the SB800. Camera in A mode (1/200 at f8) with no exposure compensation, flash set to TTL, all PW set to default.
First, using the Flex by itself, no AC3. Shoot outdoor scene with shadowed balcony in the foreground. Both foreground and background are properly exposed. Good.
Second, add the AC3 on top of the Flex. Set it to A mode and the compensation wheel to 0. This should yield the same result as without it. Shoot. The foreground is underexposed by about 1 to 1.5 stops. By increasing the AC3 wheel compensation to 1 or 2, I can get close to the same exposure as without the AC3. But I shouldn't have to do that.
I re-ran the test forward and backward with the same results: Adding the AC3 on top of the Flex with setting on A at 0 compensation yields a 1.5 stop loss of light. I tried turning camera on and off before removing it without difference.
One more piece of information. I notice that when I add the AC3 and press the shutter, the flash clearly comes on an instant BEFORE I hear the mirror flip open. It's only a fraction of a second but when you go back and forth with and without the AC3, there is clearly a difference.
And of course, everytime I add or remove the AC3 I turn off the Flex and the camera and then repower the Flex first and camera second.
Also, I re-tested this with the new firmware (3.172) and the result is the same.
That is CLT/CLS working. Before the mirror raises, a low power per-flash is fired which the camera meters to determine the power level required for the actual exposure. If that is not happening without the AC3, then something might be amiss. The AC3 is doing what you want it too.
At the top of this forum is a post dated Nov 2011 which Hal Becker made. It describes pre-flashes and the main fire command, the latter of which is not necessary for radio triggering. It is an excellent post.