One of those things that seems like it should be super easy but my mind has gone blank.
When I turn a knob on my midi controller I want to allow a sound to pass through a gate (or whatever) so the sound is audible. When I have stopped turning the knob I want the sound to stop. Almost as if I am making audible the sound of an imaginary mechanical knob.
Further development of this idea would be to map the speed at which the knob is turned to other parameters of the sound.