Pinging or knocking or detonation is basically the ignition of fuel before or after it's supposed to be ignited. What you actually hear is the explosion (fuel ignition) of the fuel against the top of the piston. If things are working as intended the explosion occurs just as the piston reaches the top of the stroke and is starting to head back down. The explosion then pushes down on the piston. This is the basic theory behind how your engine works. When things happen at the wrong time the explosion, instead of pushing down on the piston, actually hammers the piston while it's traveling upward. Now you know why it's so hard on the engine when you're pinging. If your ears were inside the engine you'd be able to hear how hard the knock is.