A line is a fold, not a mark

A frown line between the brows is skin folded by the muscles underneath pulling it in, thousands of times a year, until the crease sets into the sheet. Quieten the pull and the fold stops being made, and the skin slowly relaxes out of the groove. This is why the result builds over days rather than appearing at once, and why deep set lines soften rather than disappear. It also explains the two-sided nature of the work: the injector chooses which muscles to quieten and which to leave, because the face has to still move.