Office Mistake Proof Device

I ordered a new hole punch the other day and was pleasantly surprised to see a visual aid, that helps the user avoid mistakes, built into this device.

If your paper is aligned, the little windows on either end of the punch "light up" green. If it is not aligned, one or both ports will "light up" red. Now, it doesn't really light up - its not battery powered, so I especially like the simplicity of this visual aid.

How does it work? The only thing I could think of are those baby doll eyelids that flap up and down in a zombie-creepy manner. But this works extremely well and doesn't flop around uncontrollably. It is a simple little flapper device of some kind, so as soon as you begin to put pressure on the handle - you receive immediate feedback and prevent the error from turning into a defect.