You can say Zaraki likes to bring himself down to an opponent's level.
Unohana continues killing and reviving Zaraki in order to make him become stronger, It turns out that when Unohana encountered Zaraki when Zaraki was a kid, Zaraki was stronger than she was and wounded her. However, during their first battle Zaraki learned the pleasure of fighting to the limit as Unohana was the first opponent able to give him a challenge, and ever since then Zaraki had subconsciously limited his own powers so he can get the pleasure from fighting at his limit. Unohana's tasks is now to make Zaraki slowly release this limit and show his full potential. As the fight goes on, Zaraki fares better and better until he finally gets an opening to strike down Unohana.
The explanation about Zaraki's power sort of makes sense. He's supposedly this badass with unparalleled potential, but at the same time he is also the same guy who lost to Shikai Ichigo. Zaraki has really been handicapping himself to get the pleasure of the fight over the years. At the end of Unohana's training Zaraki will probably go back to his original strength when he was stronger than Unohana, but that's probably not his limit since he still doesn't have bankai. I'm looking forward to the chapter when Zaraki shows his true potential. I'm starting to believe the fan theory that Zaraki is really a Saiyan: he loves a good fight (must be a competitive, grueling battle) and has spiky hair to boot lol.
The bad thing here is Unohana is either going to die or at least get hurt pretty seriously. I hope she lives, but in this final arc Kubo seems to be inclined to kill off characters.