this is true, the spark does little more than convert electrical energy into thermal energy. however, the greater the electrical energy (watts) of same amount of electricity, the more thermal energy is released by the spark. it all goes back to spark duration versus spark power. a longer lasting cooler spark (less watts but lasts longer) will still generate the same amount of thermal energy over it's duration, but any heat released past the initial combusion of the gases closest to the spark will be lost to the incombustable exhaust gases that have taken their place. a short, hotter spark (more watts, shorter life) will transfer the same amount of thermal energy into those surrounding gases much more rapidly, meaning that more of the sparks heat gets into the air/fuel before it is burnt.

this increase in thermal energy translates into the flame front (i really dont see what's technical or confusing about that term), meaning that the expanding front of burning gas will expand to fill the space much faster than a cooler front. why? as the flame front expands, it heats the unburnt gases directly in front of it until those gases reach a certain temperature at which they ignite (the flashpoint). this process of heating and igniting is continuous, to use a crude example: imagine a brush fire spreaidng thru a field of grass. as the fire spreads, the grass at the perimeter of the flames is heated until it catches fire as well and the fire grows. why this means a hotter front moves faster is simple, it just takes less time for a hotter front to heat the gases at its outer edges to thier flashpoint. much like a hotter brush fire will heat the grass to its flashpoint faster than a cooler fire.

this faster moving front causes the air/fuel mixture to be burnt into exhaust gases much faster than a cooler front generated by a cooler spark would. this means that, in the same amount of time and in the same amount of space as normal (with a normal plug) there is more pressure generated by the rapidly expanding exhaust gases. this increase in gas pressure is the cause of the increase in cylinder pressure. from here, the link from higher cylinder pressure to what should be greater power is obvious enough.