The PGM Mix-Minus enables you to simply create a mix without a source or sources. Instead of using the Aux Bus to create a mix where you chose what you want in the mix, you choose what you want out of the mix.
To enable this feature, open the main settings window by clicking the icon.
Select the Audio tab.
Scroll down to the Audio Bus section. In the default mode of operation, you use the tick boxes to add in sources to your mix. Tick the PGM Mix-minus box for the appropriate Aux you wish to use. Click Apply afterwards for this change to take effect.
With nothing selected, every audio source that is in Program will be sent out live on Aux 1. But lets say that we don't want our Guest Mic in this mix because this mix will be sent back to the guest. Click on the Guest Mic box under Aux 1 to remove it from the mix. You should see a minus symbol instead of a tick. If not, make sure PGM Mix-Minus is selected and click Apply.
An easy first check to make sure this is working would be to monitor the aux levels in the software. Tick display in Audio Mixer to see what's going on. Click Apply!
As you can see here, the Aux is a lower level than the Program audio because that mix doesn't have the Guest Mic in.
If you were to remove the Studio Feed audio source too, you would see the Aux level drop to nothing because you have removed the only two sources from the mix.
If you wanted to have some Auxes operate as a PGM Mix-Minus and some in the usual way, where you select what sources you want in the mix, a combination is also possible. As you can see here;