What does the IMPD button do on the AP-66 model?

The IMPD stands for impedance, and the button alters the input impedance of the physical mic preamp circuit, similar to the Impedance switch found on Neve modules. When Lo-Z is on, input impedance is 300 ohms; when Lo-Z is off, impedance is 1200 ohms.

The impedance setting usually has a pretty subtle effect on the audio, but can have the most pronounced effect when lower impedance ribbon mics are connected.