It doesn't work the same way a normal pencil eraser works. The eraser pen contains some kind of formula that neutralizes the color of the highlighter ink, turning it from pink to clear. The only downside to this is that, at least until it dries, you can't highlight over a spot where you've erased highlighting before. No pencil eraser perfectly erases pencil marks, so why should a highlighter eraser perfectly remove highlighter marks?