Spare state is a built-in feature into any FL Studio plugin inside the FL Wrapper. It can easily be accessed on any plugin in the top left menu.
Watch the video, or you can see a detailed write-up with images on the blog:
You definitely want to use SPARE STATE in FL Studio when using plugins like compressors and EQs to make the best mixing decisions. It allows you to do A/B Comparison, and it's important you have a FAIR LEVEL COMPARISON as well!