Transient shaper
A dynamics processor that boosts or tames the attack and the tail of a sound, regardless of how loudly it was played.
Two envelope followers run side by side, one fast and one slow. Where the fast one leads, a transient is happening; where the slow one lingers, that is sustain. The difference drives the gain, so there is no threshold to set — a soft hit and a hard one get the same treatment, unlike with a compressor.
German company SPL launched the Transient Designer in the late 1990s and turned two knobs, Attack and Sustain, into the standard interface. The usual victim is drums: more click on a kick that is drowning, less room on a snare tracked in an overly lively space. Turning up attack raises the crest factor, which makes it one of the few tools in the chain that pushes in the opposite direction from everything else.