I don't know how it works, but I've never seen a fan with speed control that only had two wires.
Sure ya have, we plug two wire fans into our mobos fan headers all the time and we can control the fanspeed in the bios on most boards.
What i "think" happens is, the temperature is monitored, and there are slight voltage increases/decreases to control fanspeed based on temp.
So the fanspeed is estimated based on voltage.
So like in
Ratman6161s case there, even though he unplugged the cards fans, he still gets reports for fanspeed is because the volts on the cards fan headers are still going up and down based on temps.
So im guessing...
3 wires = real fan speed rpm monitoring
2 wires = estimated fanspeed rpm monitoring based on voltage
I have no idea how actually it works either, so this is all just a guess.
So if someone actually knows for sure and says my post is BS, then it probably is....LOL