The IK Multimedia T-RackS Bus Compressor is based on the mix bus VCA type compressor of a worldwide known and acknowledged British large format mixing console from the 80's. It was used on practically every hit record and major production from the 80's to present days, so much that its sound has almost become a trademark. This compressor was so widely used that it later became a stand-alone rackmount unit, so that it could be used separately from the large format console. This made for even more success; a thing that led to many hardware and software clones trying to emulate it.
This legendary processor has been painstakingly analyzed and measured down to the very single component, compression curve, soft knee behavior and detection circuit; this in order to obtain a dead on reproduction that will impart a cohesiveness to mixes that is often referred to as "glue effect" where every instrument blends nicely with the others, and mixed tracks suddenly start to sound like "music".
Like many of IK's hardware based modules they have added several nice and handy features not present on the original; in this case IK added the "Grit" switch that will reproduce the slightly increased 2nd harmonic distortion of an out of calibration unit, a behavior favored by many engineers for its more characterful sound.
The Sidechain HP filter has been added to filter low frequencies out of the detection circuit path so that they wont overmodulate compression, thus avoiding the "pumping" effect at lower threshold and higher ratios. When not engaged though the “pumping” effect delivered by this module is astonishing, making it equally at home on loud rock mixes or slamming dance tracks.
The 1.5:1 ratio has been added for subtle and even compression making mixes "shine" without sounding compressed.
This should sit on the mix bus of your DAW, where it was ideally designed to be, but it also rules on other stereo sources, like drums submixes or parallel compression buses: this use is also well known to engineers and producers the world over, a very effective for “obscene” amounts of compression, punching and pumping drum rooms and creative effects.