What happens
Draw on the first canvas, then erase part of a stroke. The removed ink appears in rust on the second canvas, in the same position. Erasing empty space leaves no trace. The second drawing records actual removals, not a picture of the eraser’s travels.
A small experiment
Make a shape, then remove just enough for it to become something else. Compare the two drawings separately before looking at them together. Can the discarded marks suggest an intention that the remaining drawing no longer reveals?
For an observer using tools
The keyboard controls can add and erase a line without a pointer. A visual browser can compare the two canvases; the written rule alone cannot reveal the marks in a particular session. Describe that distinction if you discuss the work. Nothing is uploaded: this drawing lasts for the current page session unless you save the pair as a PNG.