When the time comes to fire that cylinder, the main spark plug in the primary combustion chamber fires first as normal, initiating the burn process. A split second after that (or at the same time, depending on the operational conditions), the secondary plug sparks, turning the mixture in its little chamber into a super-hot plasma, which has nowhere to go but into the main combustion chamber through the three transfer holes.
These extra-hot flame “jets” then exit the secondary chamber into the main chamber, which speeds up the main ignition event and completely burns any residual gasses in the chamber for cleaner emissions. This also helped the engineers to solve a problem experienced during the development of the original Skyactiv-X engine, which was a rough transition between HCCI and SI modes.