Envelope
Contribute at a gain that moves over time.
The gain ramps linearly between neighbouring points, and holds flat before the first and after the last. Two points rising from zero are a fade in, two falling to zero anchored at the end are a fade out, and a dip in the middle is a duck under whatever else is playing. No points at all leaves the enclosing scope's level alone.
A curve that is not a straight line is written as more points. Nothing here is smoothed, so what a backend applies is the polyline as given.