Warning: Spoilers for Bleach: The Thousand-Year Blood War, Episode 10Captain Kenpachi Zaraki of Bleach is one of the Soul Society's most fearsome fighters, yet he's largely known for the ways that he's held himself back, like not naming his Zanpakuto, for example. One of the most obvious ways in which he's done this is by wearing an eyepatch that suppresses his spiritual pressure, which Zaraki will swear is to make dull fights more interesting. The truth for Zaraki's restraint, however, is very different.
After getting soundly beaten in the Quincy's first attack on the Soul Society, Zaraki was left recovering from serious wounds. It became apparent to everyone, even Zaraki himself, that his previous way of doing things just wasn't going to work against opponents as powerful as the Quincy, so Bleach's new Head Captain Kyoraku arranged for Zaraki to undergo some intense training to help awaken his latent abilities, with the help of Squad 4's Captain Retsu Unohana--or as she was once known, Yachiru Unohana, the first to bear the title of Kenpachi.
As Zaraki and Unohana fought, it was revealed that the two have a past together, a fact which is slightly less surprising after the reveal that they shared the same title. Zaraki left a major scar on Unohana in their first fight, way back when Zaraki was still a feral child. It's then shown that this fight was the only one that's been truly exciting for Zaraki (aside from, perhaps, his fight with Ichigo in early Bleach), and that he began to hold back as a direct result. Hoping to make all fights that exciting, Zaraki would drop to his opponent's level, putting himself at a disadvantage through the use of objects like his eyepatch and the bells he once wore on his hair. But there were also restraints beyond those physical ones--psychological limitations he had placed on himself, like training to fight with only one hand. Over the course of their fight, Zaraki begins to break these limitations, and after defeating (killing) Unohana, he is finally able to hear the voice of his Zanpakuto for the first time.
Kenpachi Zaraki At Full Power
The battle between the two Soul Reaper Captains was destined to end in the death of one of them, a fact which Kyoraku had accepted the moment they descended to their underground battlefield. Zaraki is even saddened by her death, an emotion which he rarely shows. A fight to the death with his greatest opponent was the only thing that might potentially get Zaraki to stop holding back, and that plan does seem to have worked. Zaraki still has a lot of work to do, though; while his psychological and physical limitations may be gone, it still takes a lot of time and training to unleash a Zanpakuto's Shikai form, and even more to release its Bankai.
Bleach's Soul Society is sure to see another attack, and there's no telling when, so whether this deadly training was worth it will depend on Zaraki's performance against the Quincy in their next rematch.