Cursed Seal of Reflection

The Cursed Seal of Reflection (自省の呪印, Jisei no Juin) is a inscribed unto Mukūrō ever since he was young in order to control the extent of his powers. However, it is only in recent years, through tutelage underneath Kazuhide and, to a certain extent, Akiya.

Description
This particular was one inscribed into his being within  following the realization of the nature of Mukūrō's powers. The seal itself began at the core of his, which is around the stomach. In a similar manner to the, the seal blocked all of the access to the exertion of chakra from this particular region, thereby thinning out his abilities extensively and forcing him significant fatigue over the course of his early life. However, the seal was designed by its wielder to be manipulated once the inflicted possessed enough control over their chakra which, as fate intended, would be a successfully passed down intention given Mukūrō's encounters.

At this point in time, Mukūrō exclusively makes use of the seal to block his chakra absorption at his gut, like he did during his youth. Thus, no absorbed chakra can be refined entirely until he wills it — hence the frequency of his chakra pulses. However, at any point in time, he can remove the seal to access a greater reservoir of power. Alternatively, he can activate the Juinjutsu to block the movement of chakra to a certain location and actually send it outside of his body at a rapid rate, thereby evading fatal repercussions of certain chakra enhancements. When performing this function, the entirety of Mukūrō's become dyed black.

Trivia

 * While the word reflection (自省, Jisei) is used in the seal, it is actually referring to self-examination. While the author wished to use the alternate reading meaning self-control (自制, Jisei), it impeded with the flow of the naming, hence why the former was used.