Expanding on Compression: 3 Overlooked Techniques for Improving Dynamic Range:

“When many engineers say ‘compression’, what they mean is “downward compression.” In other words, bringing down the level of the signal above the threshold that you set on your compressor, to make louder things quieter. But all too often, we forget about upward compression, where quieter sounds are brought up to the threshold point; this technique can be quite handy in certain situations for a more transparent effect (it can also be approximated with parallel compression, if you don’t have an upward-compressor on hand).

Reference pointer for my Compression post coming up.