... maybe on a older slacker engin'.
Not necessarily! - it could happen to one of any age. (But I don't want to start a panic).
I spoke to one on our mechanics at work today and he confirmed my theory. The point of ignition in the compression stroke is a few degrees before the piston reaches Top-Dead-Centre. Normally, the piston would continue up to TDC and beyond into the power stroke by either the continuing assistance of the starter motor or the power stroke of the previous cylinder in the firing sequence.
In your case, there was no power stroke from previously as the engine was yet to fire and the starter motor was disengaged early. As a result, just as the piston of the first cylinder to fire reached the point of ignition, it effectively 'bounced' back against the compression and fired in reverse. This power stroke, albeit in the wrong direction would be enough to bring the next cylinder in the now reversed sequence to the point of ignition and so on and so on, many times per second.
The engine is now running, but backwards, and because the phasing for fuel injection and the valve timings are now out of sync, this is why the engine feels like it is about to self-destruct, and given enough time (and we're not talking very long here
), that's exactly what most likely would happen. At the very least, it would probably 'mince' the tensioners and so throw the timing belt.
So the moral of the story is: "keep your finger on the start button until she's fully fired up, but if the 'scary stuff' ever occurs again, shut it down immediately". Hope you don't have too many sleepless nights worrying about this but I am assured that it is quite a rare event.