Internal combustion engines, like yours, can produce the last portion of the combustion externally.
This happens mainly for rich mixes of air and fuel, in which the propagation of the flame is much slower than for a proper mix.
That flame starts at the plug and propagates like a front wave thru the combustion chamber until it reaches the head of the piston.
Ideally, that should happen before the piston is forced down by the high pressure gasses and it opens the exhaust port.