Basically what it allows you to do is turn off each drum sound or the VCO individually, handy for jamming. Note that the snare has 2 switches, one for the shell and one for the noise, quite handy if you have my drum mod board as it allows a few extra sounds like hats, clap, rim and conga type sounds depending on the position of the drum mod and mute switches. For example with the noise turned off and the drum mod snare set to high it sounds like a conga etc. The VCO mute allows you to do filtered noise or external input without hearing the VCO. Operation is simple, when switches are down they are off, when up the sound will be heard.
The drum mix out provides a separate output for the drum section, it is independant of the drum volume control, meaning that the drums can be turned down but still available at the drum mix output, so that you can have external fx on the drums or synth section separately, or if you want you can run the drums back into the audio in for filtering.
Please note the drum output is not quite as loud as the main output, but when you run into a mixer it is not a problem. Price is £30GBP plus postage, if you want to order get in contact by email link at top right of page.
Here is a picture