Yu Mei had only partially unlocked the heart mirror, however, the partial features were sufficient to give her team a leg up in gaining ground.
The heart mirror was similar to a spiritual sense in that it could be used to detect the surroundings on a detailed level and wasn't affected by the concept of darkness and light.
However, there were a few differences between the spiritual sense and the heart mirror, one of which was versatility. The spiritual sense was much more versatile than the heart mirror when it came to using it outside of combat. It could be used to investigate ores, artifacts, and weapons down to researching the structure of herbs and spiritual plants.