Parallel Compression is a mixing technique aimed to reduce the dynamic range of an audio signal. It’s also commonly referred to as New York Compression.
Normally when we’re talking about compression, we’re talking about downwards compression. This is when we make the louder parts of an audio signal quieter, reducing the dynamic range so we can increase the overall volume.
Another way to reduce dynamic range is to use upwards compression. This is when we make the quieter parts of our audio signal louder, accomplishing the same results.
Parallel Compression however is just a slightly different way to practice upwards compression. But instead of using a compressor to apply upwards compression directly to our audio signal, we instead duplicate our audio signal and apply heavy downwards compression to that duplicate and proceed to mix it back in with our untouched original signal.
The overall results will indeed reduce the dynamic range, but without sacrificing our delicate transients which can sometimes get obliterated when trying to downward compress our way through the loudness war.