Well, I got my explanation from grabbing an oscilloscope and probing that line from the ECM to the FCM and figuring out that they were using a pulse width modulation technique to tell the FCM what speed to run the fan. (The FCM also uses PWM to vary the fan speed - but that's not the point here.) (This after Rudy told me part of it, back in 2009 - I then started my own investigation cause I can't NOT know!)

I could see the pulses going back and forth during times when the ECM was not telling the fan to run at all - you could easily see the handshake.