The sidechain allows you to filter (within the program, not an external sidechain) above or below certain frequencies.
For example, if you use a low cut filter set at 400Hz, you will be reducing the amount of low frequency going into your compressor and thus the amount of your compression will be less. Thus, you should hear a little more bass compared to if you had that low cut filter set to 50Hz.
The same laws would apply in apply low pass, high cut and high pass filters.
You can use the Mon button to hear what your sidechain input sounds like. This automatically switches off when you close the GUI probably to avoid forgetting to turn it off.

I still have not figured out stereo linking yet. I guess this would be useful, say if you had your bass panned to the left and then had your low cut filter set at 400Hz. So now in your output, after compression, you would have greater volume on your left side because it was compressed less.
I imagine that the linking options and tilting option would allow you to offset this stereo shift, although I would not know how to really implement this.

Anybody have further clarification and guidance on this?