A matrix is a portrait of a transformation: its columns are exactly where the basis vectors e₁ and e₂ land, so dragging the two gold handles is editing the matrix.
The gold cell is the image of the unit square, and its signed area is the determinant — the factor by which every patch of the plane scales, all at once.
When the two columns line up, the column box collapses: det = 0, the whole plane is crushed onto one line, and different landmarks pile onto the same image point.
That is why the undo lever jams at det = 0 — once many points share one image, no function can send one point back to many. Singular means irreversible.
The press seized mid-print. The lesson continues without it — everything you have already done is safe.