Can you connect a 5V PWM (5V input current, such as Noctua's NF-A20 5V PWM) fan directly to the part even though 12 volts are "pulled" from the motherboard?
Does the NA-FC1 regulate the current accordingly?

Question

Can you connect a 5V PWM (5V input current, such as Noctua's NF-A20 5V PWM) fan directly to the part even though 12 volts are "pulled" from the motherboard? 
Does the NA-FC1 regulate the current accordingly?

Answer

Yes, you can.
https://noctua.at/de/wie-kann-der-na-fc1-zur-regelung-von-usb-betriebenen-5v-pwm-lftern-verwendet-werden

Answer

No. The speed signal is controlled and not the voltage. With a 5V PWM fan at the output, 5V must also be supplied at the input, otherwise the fan is ruined. For the 5V PWM fan there is a USB cable and SATA cable which has the 4 pin fan connector. See details on the Noctua page. For feeding from the mainboard, please use a 12V PWM fan.

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Question about Noctua NA-FC1 Fan Controller

Can you connect a 5V PWM (5V input current, such as Noctua's NF-A20 5V PWM) fan directly to the part even though 12 volts are "pulled" from the motherboard? Does the NA-FC1 regulate the current accordingly?