Compression is a very useful tool within mixing, as it allows you to be able to fill out your mix while still keeping the actual loudness to a reasonable volume. How it does this, is it “reduces the dynamic range of the input signal. Fundamentally [compression] exists to reduce the overall dynamic range from something large and unmanageable to something smaller and more appropriate for the intended application (Robjohns, 2013).”
Most compressors work on the basis of bringing the loudest signals down, rather than the lowest signals up. However, parallel compression is a way of, “[leaving] delicate loud transients intact while raising the level of low-level signals (Robjohns, 2013).” So how does this work? The basic concept is to begin with two identical tracks. Leave the first one alone, and compress the second one. You then mix your compressed signal with your dry signal to produce the parallel compression (The LA Recordist, 2016).
To do this, the ratio needs to be very high (so as to almost make it like a limiter), and the threshold needs to be adjusted in order to supply a lot of gain reduction when the input signal is the highest. Something to be wary of in this situation is that sometimes compressors will generate a distortion when you apply a lot of gain reduction, it just depends on your compressor (Robjohns, 2013). For an example see below:
So basically this technique raises the quiet bits of your audio by 6dB, while leaving the loudness how it is. This is because there is obviously not going to be any gain reduction on them, and therefore you are just mixing two identical signals together. If you need more reduction on your audio, it is suggested to layer more parallel compression over it (Robjohns, 2013). Another experimental thing you can do with parallel compression is to raise the volume of your compressed track. Watch the video below for a really simple, visual representation of parallel compression:
Robjohns, H. (2013). Parallel Compression. Soundonsound.com. Retrieved 3 March 2017, from http://www.soundonsound.com/techniques/parallel-compression
The LA Recordist,. (2016). Parallel Compression Explained Visually. Retrieved from https://www.youtube.com/watch?v=3K9DaROkeK4