Here's a photo of the stock piston showing the slot feature.
https://image.ibb.co/hqLa9H/IMGP0288.jpg
Very illustrative photo!
To further clarify my post above, it's the heat that causes these failures.
Excessive heat causes rapid onset of detonation, due to preheating of the incoming mixture. When detonation, or pre-ignition occurs, the protective air boundary layer on top of the piston is blown away, and the alloy is exposed directly to the combustion temperature, which it cannot stand for long. It absorbs too much heat, expands beyond the clearance in the cylinder, and rubs the walls. This pushes any remaining oil away, and it seizes. If you are lucky, it only mini-seizes, and you stop for a little while, and it will start back up again. This doesn't mean that nothing is damaged, but merely that the damage was somewhat minor. That's if you are lucky.
If you are not lucky, the piston will separate, and the engine will beat it's internal parts with the loose con rod. If the con rod breaks instead, which is also common, then the stub of the broken con rod will flail around inside the engine case, knocking big holes in it, and engine parts and oil ejecting all over the road.
This happens most commonly when riding at low rpms, encountering a hill, and lugging up the hill in top gear, thinking everything is fine. The pinging begins, the piston heats up, and.....BANG. The engine comes apart. I have engine cases here with holes in them from exactly that.
That is why the rpms need to stay in the "happy" range that I stated previously. Use the gearbox.