But the power reductions on the transmitter combine with the cuts on the receiving flash to reduce the output on the receiving flash. So if I have the receiving flash set at 1/1 power on the unit, and then I lower power to its group (a,b or c) on the transmitter to say 1/4, it shoots at 1/4. But if I set the unit at 1/4 and reduce its group to 1/8 on the transmitter, the power emitted is 1/32.
So a single unit lets you knock 7 stops off of full power (1/128 is the lowest power setting). But if you fire the flash set at 1/128 on the unit itself, and the transmitter is also set to 1/128, you get a flash of 1/16,384 power, which is (I think) a 14 stop reduction. Or more practical, you could have one on 1/2 and the other on 1/128 for a more usable 1/256 power.