As the name "alert in the shadows" suggested, the fighting style was keeping one's senses in shadows, in this case a drow's natural ability to cast darkness. It was special because it wasn't about the usage of specific moves or weapons.
Drow in general used their darkness ability for deception and battlefield control purposes during combat. A user of this style fought within the darkness while remaining more alert and aware about the surrounding than an enemy, thus limiting the own disadvantages to fight in the darkness while mitigating the advantages of the enemy in a fight in the sightless field.
As the name suggested, would-be users needed to be more alert to pinpoint the location of enemies while being blind.
The style was based on the Blind-Fight and Alertness feat in 3.0-edition Dungeons & Dragons.