You use it if you smudge the polish. You just go over the smudged bit a number of times and it kind of 'melts' the polish a bit and allows you re-manipulate it to cover over where you've smudged it.
I've used it on myself odd times, but have found it depends on how badly you smudge the polish as to how effective it is, sometimes I've just ended removing the polish and re-doing that nail.
I did find you need quite a few strokes of the chip skip as well before anything happened.