In the words of Keeley Effects:

The DynaTrem Dynamic Tremolo by Keeley Engineering is an all new type
of tremolo that is dynamic and responsive to your playing.Your
playing controls the Rate and Depth.The harder you play, the more it
responds.Play softly and it relaxes.Set the sensitivity with Rate or
Depth controls and you have an unparalleled ability to be expressive
with your guitar and tremolo.