Thanks for the explanation. If i understand correctly, this means that all the fan does for communicating the RPM, is to short the tachometer wire to ground twice per rotation, which causes the voltage on the reading Arduino pin to drop, which is an event that i can use to trigger an interrupt.
This would also mean that the voltage is "supplied" by the Arduino itself, so the pin receives a safe 3.3V when the transistor is open and 0V when the transistor is closed.