Crushed blacks

Blacks crush when the darkest parts of an image land at or below the bottom of the recorded range, so what were separate shades all read as one solid black. It happens in camera, through underexposure or a heavy picture profile, and in the grade, by pulling the shadow end of a curve too far down. The detail is gone. There is no data left to lift, and no software recovers what was never recorded. Some crushing is a choice, since high-contrast looks and moody night scenes want it. What you want to avoid is crushing by accident and finding that a face, a jacket or a whole background has flattened into a shape.