After reading my post I think I need to clarify a point. Combustion does start before TDC, but cylinder pressure, in a properly tuned engine, will not become significant until ADC.
It takes a while for the combustion front to reach all of the fuel mixture, so cylinder pressure looks kind of like a geometric curve.
You don't want cylinder pressure higher than atmospheric pressure until after TDC. Otherwise, the cylinder pressure will try to push the cylinder back down instead of letting it pass through TDC. Nothing good comes of that.
