What the stretch keeps still

Every contrast move holds one tonal level still, and where that level sits decides what survives.

Contrast and Mood named the pivot: the tonal level that holds relatively steady while darker values move down and brighter values move up. It is a control, and you move it yourself. In Resolve it sits beside Contrast in the Primaries palette and it is called Pivot. On this bench it is the slider marked PIVOT.

The rule is simpler than it sounds. The pivot should sit near the tone you want to protect. Blackmagic states it directly. Darker images may need a lower Pivot value so the stretch does not crush the shadows too much. Lighter images may benefit from a higher one to get shadow density back.

Take a night exterior. Almost everything in it already sits low, so a default pivot puts the stretch above the part of the picture you care about. The shadows go down, the floor of the trace flattens, and the one lamp gets brighter for no benefit. Lower the pivot and the same amount of contrast opens the picture instead of crushing it.

Do not memorize a number. No published value exists, and one borrowed from another timeline will be wrong on yours. The value that holds a particular tone moves with the picture and with the way the footage is encoded. That is why working colorists quote different pivots and each of them is right about their own timeline.

Watch the floor of the trace while you move it. Raise the pivot and more of the picture sits below it, so more of the picture gets pushed down. Lower the pivot and the same contrast setting leaves more of the frame above it.

There is a third answer and it is often the correct one. Use less contrast. If a picture only reaches the ratio you want at a setting that flattens its floor, that ratio is not in the plate. Force it and you lose shadow detail you cannot get back.

A custom curve makes the same decision by putting a control point on the tone you want to protect and moving the rest around it. The Working Grade's curves room teaches that instrument.