Ableton's official Learn Live series covers Gate and Ducking, the two threshold-based dynamics controls inside the Echo device's Character section. Both share the same core mechanism: a threshold value that determines when the parameter activates, plus a Release Time that controls how long the effect lingers.
The Gate filters which incoming signals actually get delayed. Only audio that rises above the set threshold passes through to the delay engine, and the Release Time shapes how quickly the gate closes after the signal drops back down.
Ducking works in the opposite direction: when the input signal exceeds the threshold, the delay output is temporarily turned down. This keeps the wet signal out of the way during loud or busy passages, then lets it rise back up in the gaps.