Blood Vessel Manipulation
Rui most certainly recalled the Martial Heart manipulation technique that the Gatekeeper had deployed in their battle. The man had exquisite control over his Martial Heart, which allowed him to use it in extremely flexible and efficient manners. Rui hadn't even considered the technique because he assumed that it was extremely difficult, making it utterly non-viable for standard Martial Artists.
However, according to the Gatekeeper, it was something that powerful because of his mastery of the technique rather than the technique itself. It also meant that the technique difficulty was lower, making it viable for Rui to have it serve as a method of bodily Adaptive Evolution for those who joined the Water Sect in hopes of mastering it to a greater degree.