This is a simple little plug-in that does one thing extremely well - it gets other stuff out of the way. It does that by employing a unique and elegant method, creating a reverse EQ profile of whatever signal you feed into its sidechain input. It works like this: when you assign TrackSpacer a sidechain signal (via an internal bus or external hardware input), you can see a rough spectrogram in blue bouncing around on the main page of the GUI. Underneath that, you can see a white curve reacting inversely, which roughly shows the "gain reduction" being applied to whatever track the plug-in is placed on - except that gain reduction is happening individually on 32 separate slices of the frequency spectrum. In other words, as a given frequency, say 500 Hz, increases in amplitude in your sidechain signal, that frequency gets proportionally "ducked" from your main signal. The amount of this frequency-dependent ducking is determined by a large ratio knob, from 0-100%.
Wavesfactory claims this isn't really multi-band compression, since it has 32 bands instead of the normal three to five, but it acts a little like a compressor, which is belied by a couple of controls on the Advanced control panel - you can set attack and release times to determine how quickly the subtractive EQ is being applied, and how long it stays in effect after the sidechain signal goes away. Other Advanced controls are switching from L/R operation to M/S operation, and a pan pot to determine whether the effect is happening more on the left or right (or mid or side) channel. M/S operation is great for, say, having a bass drum clean up the low frequencies in the center of a stereo pair of room mics, without affecting the EQ of the outer edges. Or vice-versa.
Back on the main panel, you can use high and low-pass filters to exclude low or high frequencies (respectively) from triggering TrackSpacer. Using these controls, you could dial in just a small range of notes on your source track to take effect on your TrackSpace'd signal. You can audition this sidechain on the Advanced panel. (Of course, the audition function only works properly if your control track is muted and feeding the sidechain via a pre-fader aux send, but I digress.) Also on the main panel is a Freeze button, which captures a subtractive EQ profile for static application onto the affected track.
I would call this a secret-weapon plug-in, but the word is hereby out! TrackSpacer is a ridiculously affordable, easy way to get various elements of your mix to interact in useful and musical ways. Be careful not to abuse it, though - unless you do so creatively - because it's astonishingly seductive at first. The temptation to have the lead vocal subtracting its EQ profile at all times from every other bus in the session pays off with a nice, forward vocal that isn't overly loud. Just make sure your other elements aren't suffering, the way they were on the first couple of mixes I employed TrackSpacer. If I listened closely to the guitars, for instance, I could hear too much of the body of the sounds getting subtracted when the vocal kicked in. As with all tools, even ones as straightforward as TrackSpacer, there is a learning curve to effective and transparent use. But now, a couple of months in, I am more judicious and confident in my use of the plug-in (largely in part to the M/S and filter controls), and I rely on it heavily for certain tasks that used to be much more laborious. Luckily it's available in AAX 64bit, as well as VST, VST3, AU, and RTAS, so it will stay with me through my gradual migration to Pro Tools 11, unlike some other small-manufacturer plug-ins I have grown to love. Final words: buy it! It works!