The pacemaker detects that the authors heart has stopped or is in such extreme arrhythmia that he will probably lose consciousness and die.
Working in the same way as a normal defibrillator that you might see in an Emergency Department the pacemaker supplies a large shock to the heart to restart it or get it to beat properly.
The shock is large enough so that a person will fall to the ground when they experience it. And the article mentions that if you are in a pool near somebody with one of these pacemakers then you might feel the shock as well.
To be a little more clear pacemaker != defibrillator, although often they are combined into one device. A defibrillator gives a large shock to the heart to knock it out of a dangerous rhythm such as ventricular tachycardia or ventricular fibrillation. A pacemaker is designed to send a smaller shock in such a way as to stimulate the atria and/or ventricles to depolarize and contract (recreating the normal rhythm).
Pacemakers are designed to either constantly go off or to detect missed beats and give shocks. They do not knock people down and I've never seen anybody ever complain of a pacemaker shock being even detectable. A defibrillator can be part of a pacemaker (in which case it's called an ICD), and it WILL hurt if it goes off. I think that is what you mean.
Is that a bug or a safety mechanism?