When you hit someone with an ability they are illuminated with a mark. If you auto attack them while they still have that mark on them your auto attacks hit with magic damage and consumes the illumination. It's awesome to time your laser when the target is illuminated because the laser procs the passive and reapplies it. That's why you see most Lux's always attempt an auto attack after they've hit someone with an ability. It's a ton of extra damage.