Actually it's just switching either 12v or 5v for case fan.
not much help on raising up the fan speed (RPM), but when your set to 5v will slightly slow down the RPM of the fan.

that day i saw a molex converter, output is (3pins 12v x 2) + (3pins 5v x 2). i mean 1 molex to 4 case fan output.
function is let us choose the high speed 12v or low speed 5v power supply.

here... lol

just showing your way cant make the fan to reach the highest speed, since using 12v power already running at highest speed! totally no different after modified from 3 pin to molex.

that's another story (motherboard temp control system)

old mainboard normally dont have temp control system... lol

just pointing your tips is not that useful... i mean no sense
or maybe just discussing here, i dont think my post consider as spamming!
example of spam just like what i quoted! (up there) ok?
just focus or answer my post if you want to quote my post!
and stop saying i'm spamming here, that should be a real spam if you reply like that. lol

sorry, back to topic...

on board system fan will control the fan speed (RPM) just in small gap like 2500RPM - 2900RPM... we can disable it, if some mainboard not able to disble it, i think the board should have no fan speed control system.

beside the system fan, some of the mainboard providing 'power fan'.
running the max speed all the time.

for my example,
i'm using CM 80mm case fan.
plug to system fan - 2909RPM
plug to casing fan1 or fan2 - 2900RPM

ps : casing is pwrlogic artix.