Instead of trying to explain myself here's a pic I made in 3 minutes.
Can a fan be hooked to the power supply for power and the mobo for the tach?
The reason I'm doing this is the only header I have left is PWR_FAN. If I plug a fan in there one of my chassis fans hooked to the mobo slows down, noticeably. I already wired the power to a molex end, but i left the yellow wire in the 3-pin connector.