Gist: Light a falling object with a strobe at a known rate, and every flash marks a known instant. The spacing between flashes is then a distance you can measure.
Why it matters: A Hz strobe gives two ticks per second, so the time between images is known exactly. Distance between flashes plus that known time gives speed; with known it also pins down where in the fall the object was, and so the depth. The lecture calls this “like a vision problem”.
Detail: Lecture 2, p.7
Related: Measuring g by free fall, Average velocity, Measurement resolution, Physics