Well, for starters, it's not an audio mixer...but this should explain what's actually going on...
So, you have four inputs which go "across" to the four pots. Now, if you enable the DC offset jumper, you'll have DC offsets on whichever horizontal row isn't connected to a signal via the bottom pots. This lets you set up three (one of them is for the offset) 1-out/3-in mixers which can add that CV to another CV.
Let's say you want to use it with a sequencer. So, what you'd do is to split the CV coming out of the sequencer's 1V/8va output into three identical signals via a mult. You'll have the same signal on inputs 1-3, and row 4 is your offsets. Then, after also setting the matrix mixer in bipolar mode, you'll have four different output channels with the sequencer out PLUS whatever offset you've dialed in. Couple that with a 4-in/1-out sequential switch, and you can then step through matrix outs 1-4 and let the DC offsets be used to transpose whatever pitch CV destination is desired. The nice thing about using this in bipolar mode is that you can also subtract voltages, so if you suddenly need a BIG pitch change either up or down from the sequencer's output, that's easily done.
Where this really gets fun is when you start combining modulation signals. Remember, you've got 4 ins and 4 outs...and if you want "composite" modulation signals, you can send four different modulation sources to the matrix mixer and get four different composite modulation outputs, depending on how you set the pots.
Basically, they're amazing for scrambling up mods and CVs...Jim's sort of the matrix mixer guy here, so hopefully he'll pop in and point out some other uses.