Not sure what he’s referring to, but Changing the frequency can help give you a more linear response for tuning purposes. So instead
of trying to tune in a 10% range, you get a 50-60% effective range. Above a certain percentage and below a certain percentage, idle valves won’t increase
or decrease RPMs.

The PWM Converter I mentioned is this item: PWM Converter Board (jbperf.com) If the OP's secondhand PNP has this mod fitted, then its IAC valve drive will be inverted: 100% is closed and 0% is open.

(It stops the IAC valve buzzing. The MS2 can drive the valve with a precision of up to 256 steps but, particularly with the 1.8 valve, not at a high enough frequency to stop the valve buzzing. The stock 1.8 ECU runs those at 300-400 Hz, several times faster than the 1.6 type. You can increase the MS2 frequency but the trade off is you lose precision and have fewer steps. The converter lets you have the best of both worlds.)

Thanks, will try that.