A quick look at a 400w Osram SON-T lamp with an integral ignitor.
The advantages of a self-starting SON lamp is that an external ignitor is not required and thus reduces complexity and cost of the control gear. Only a choke and PFC capacitor is required.
The huge disadvantage of course is the greatly increased cool-down time required before the lamp can restrike, the reason why internal ignitor SON lamps are rarely found these days.
Sometimes the switch can weld itself closed which can cause ballast failure.

It works similar to a fluorescent starter switch (in fact lower wattage lamps like 70w actually have a glow-bottle switches in them).
When cold, the switch shorts out the arc tube via a heater coil which when power is applied heats up a bi-metallic switch. When the switch reaches a certain temperature, it snaps open and breaks the circuit which causes a high-voltage spike from the collapsing magnetic field in the ballast. This is used to strike the arc inside the discharge tube.