The robot knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is greater), it obtains a difference, or deviation.
It's definitely floated around for a while, but it grew in popularity in the past few years.
This always stuck out in an otherwise excellent bit, because you should definitely _not_ be taking the absolute value of your control error.