What's the working principle of a painkiller pump?

I'm assuming you mean the button you push to administer pain killers after you have surgery or some other medical procedure that results in pain.  The idea is that the patient is better able to do so without waiting too long, or getting it too often.  The pumps are on a timer, so you can't OD.  It bets waiting for a nurse to show up and add it to your IV.