 A moment later the starter interrupts the current flow from the ballast, which has a high inductance and therefore tries to maintain the current flow (the ballast opposes any change in current through it); it cannot, as there is no longer a 'circuit'.As a result, a high voltage appears across the ballast momentarily, to which the lamp is connected; therefore the lamp receives this high voltage across it which 'strikes' the arc within the tube/lamp.The circuit will repeat this action until the lamp is ionized enough to sustain the arc.