It is an explosion, i.e. "rapid expansion in volume associated with an extremely vigorous outward release of energy, usually with the generation of high temperatures and release of high-pressure gases"
Pinging/knocking is when there are two or more explosions in the one cylinder and the wave fronts meet up
Its why diesels sound the way they do, they have no fixed point like a spark plug where the air/fuel is ignited
It just ignites when compression and heat are high enough and often in more that one spot on the cylinder, so pingy
Its just a small explosion from very little gasoline, not even firecracker strong I would think
But it happens in an enclosed space
Its like the old saying if you hold your hand open and let a fire cracker go off ON your hand it will sting, but if you close your hand around the fire cracker tightly and it goes off you won't be using that hand for a LONG LONG TIME, lol