Parallel compression, also known as 'New York' compression, is a technique based on mixing a dry signal with a heavily compressed identical signal. It is thought to maintain the subtleties of a performance while stabilizing the dynamics.
The mix controller of the alpha compressor makes it possible to cross-fade between the unprocessed and the compressed and filtered signals. This allows parallel compression right in the box and supersedes additional routings in favor of a better signal quality.
Now you can use even extreme compression settings without killing a track by winning the loudness war. By mixing just a part of the compressed signal to the original, the major portion of the initial dynamic structure remains intact.