Way it was described to me was that in such an instance, the heart is basically fluttering rather than the rhythmic pumping action it usually does.
In the old days, you'd see someone pounding on the chest of the victim, the theory being that the jolt of the thump would stop the hearts "wobble" and get it back into it's usual pace.
The CPR instructor who was talking about this, basically said calling for EMTs was ALWAYS the best option and that you were just as, if not more likely to just stop the heart dead (no pun intended) by the thump than you would be to settling it back, ticking over again.
In a moment of grim sharing, she did go on to say that if you were out in the middle of nowhere, no chance that any rescue teams would get to you in any kind of useful time-frame, you could try it - it would either have the person up on their feet again (other injuries not withstanding) or you could focus on just looking after yourself as the victim would no longer need your attention.