X-Limit is a rare trait that is rarer than Haoshoku haki that requires a spirtit with an unbending will. The user of this technique can utilize the powers of their Devil Fruit to forcably override their bodies maximum limits. This is usually done only during extremely dire situations as the user can be killed do to stress or at the very least put into a serious coma. It was first deiscovered by Zero the White Frost during the great war with Grey Ragnarok. He was able to teach this technique to every sentinel and the secret was passes on down the generations until it was forgotten. The Sentinels were the only "race" to master this technique completely. However a select few have also unlocked this power. Two of them being Monkey D. Luffy whos gear's two and three can be considered X-Limit and Magellan whos Jigoku poison qualifies aswell. X-Limit in and of itself boosts the users Devil Fruit abilities 10 to 100 to even 1000 fold. But in excahage can kill the user if used incorrectly however noone has died yert of it's use though many of the sentinels were knocked unconsious and put ino a coma for weeks after prolonged use. Using x-Limit even once taxes the human mind so considerably that only true masters of the art can even remain consious after it's first use. Using it twice in one day was unheard of until Train d. Oversoul was forced to use it as a last resort. If the initial use of both X-Limits doesn't kill you it will shorten your life span by as few as 35 years which was why Oversoul died at such a young age.