Basically, it is a normal state-variable filter. The twist is that the filter frequency can be controlled by MIDI note number in addition to the normal knob-twisting action. You instansiate it as an insert effect (or send, if you feel daring), then assign a MIDI track to it the same way you would to, say, a VSTi or other MIDI-capable effect. You can then "play" the filter with your MIDI keyboard. After using it for a while, I added the "hang" button to the MIDI control section. If this is on, the filter stays at whatever the last MIDI note was. If it is off, the filter returns to the knob value as soon as the key is released. If you use it in this mode, you'll want to quantize the ends of the notes, as well, unless your timing is absolutely stellar.