Magic based! Demons in the Nasuverse feed off of emotions and Lancelot's sword, Arondight, is already a demonic weapon.

So, his Exsi version of it is going to give Lancelot magical energy based off of the type of emotion prevalent in the air. It's going to turn Dark if it's negative emotion--Light if it's positive, etc.

Plus, Lancelot's literally made of magic.