How does this code change a button's background color when clicked?

Question

The following code gotten from this post supposedly changes a button's color when it is pressed.

  override var highlighted: Bool {
        didSet {
            if highlighted {
                backgroundColor = UIColor.lightGrayColor()
            } else {
                backgroundColor = UIColor.whiteColor()
            }
        }
    }

However, I was wondering what the variable "highlighted" would corrospond too?

If I programatically added a button to my view like this:

let exampleButton = UIButton()
self.view.addSubview(exampleButton)
//Set constraints,title-text,etc... 

How would I get this to work with exampleButton?