One idea could be that the mate mark is a hidden seal that only reveals itself when they are in true danger. It serves as a connection that allows them to share chakra and strength, enhancing their abilities when they fight together.
Perhaps the mate mark is a symbol on their wrists that glows when they think of each other. In fanfiction, it could be used to show the deep emotional bond between Naruto and Hinata. For example, when Naruto is on a long mission, Hinata can look at her mark and feel closer to him, and vice versa.