Side Chain Compressor was developed by Slim Slow Slider for VST windows.
apulSoft ported this plugin to OSX and to the AU format.

This is a Side Chain only compressor which requires special routing.
For VST there are two versions, mono (2 input channels/1 output channel) and stereo (4 input channels/2 output channels). Not all vst hosts support the required routing - Cubase Studio for instants does not. The AU version uses the regular AU sidechain mechanism and detects mono vs. stereo processing.