This is how a piston looks around 10-15 degrees before top dead center. With initial timing set at 10-15 degrees BTDC, the mixture fires before the piston reaches top dead center. There's a reason for this. Contrary to popular belief, the fuel mixture does not explode in the chamber. It reacts in more of a quick fire that flashes across the top of the piston, making heat energy that acts on the piston. Because it takes time for fuel to ignite and make heat energy, we need to light it before the piston reaches top dead center. By the time the piston reaches top dead center, it's ready to move downward, turning the crankshaft and making rotary motion.