Set up a clip on the timeline with a keyframe curve that fades down to zero, so basically with four keyframes: 0, 100, 100, 0.

Drag one of the trimming handles: if you drag the beginning's trim handle to the right (or if you drag the end's trim handle to the left), you will see one of the zeroes become something else inbetween zero and 100.

Another (more funky) case is this: trim the clip's end, and THEN move its last keyframe down to zero... and then drag the end's trimming handle to the right to re-elongate the clip, and the keyframe will go into the negative, so the keyframe curve will go "through the floor".

Instead, it should stay "clamped" to whatever value it had, since the first and last keyframes are kinda special.