Parallel compression
Blending a heavily compressed copy of a signal with the original, so quiet detail comes up without flattening the transients.
A regular compressor pulls the loud parts down and blunts drum hits and consonants in the process. Parallel compression works the other way round: the original passes untouched, a copy gets crushed, and the copy is mixed back in to taste. The quiet parts rise while the peaks largely keep their shape — upward compression, strictly speaking. It is often called New York compression, after the studios where it became routine on drums.
It used to cost an extra channel on the console. Today almost every plugin has a mix knob that does the same, but the copy still has to be phase-aligned with the original or you get a comb filter. That is what delay compensation is for.