Divination
Order. Theo felt that the Order symbol in his body was like a bottomless pit. Every day, a portion of his energy actually entered the pit, filling it.
This allowed him to utilize the Order. The more understanding he had about his Order, the faster he could fill this bottomless pit.
That was why the Dragon always told him to understand his Order. It was because the energy that entered the pit would soon be enough for him to reach the next stage.
Obviously, the Lightning Authority had a far faster rate to fill that bottomless pit. After all, to reach an Authority Level Figure, his understanding of Order would have to be beyond anyone's imagination.
When the mermaid poured that energy into his Order symbol, he realized it could enter the pit. In other words, the mermaid actually had the ability to fill that bottomless pit, allowing him to use some of that energy to fasten his progress in learning about Reality and Illusion.