It started back when Umin sacrificed a portion of his soul to gain the Qi necessary to fight against the Forged Existence.
That damage to his soul created a hole in his soul that couldn't be entirely fixed. Something he himself hadn't anticipated. Despite his soul being healed, Umin could not patch this hole. Not that he needed to. Somehow this hole in his soul was dispersing a faint strand of Qi. When Igneel was in control Umin focused his attention to gathering this Qi.
With it, he had enough to utilize in combat when the time arose! Without needing to sacrifice his soul any further for temporary strength. Although it was merely a fraction of what he could do originally, being able to replicate the original techniques of the Misten Cloud Sect made him extremely happy.
