Also I'ma go ahead and take a shot.
Ironically, Souj's picture of Suzaku is also hilarious for me. Why? Because he too cannot "die" (because of the "curse" Lelouch placed on him). With that said, let me take a shot at what's going on here:
In some form or fashion, Unohana's bankai can place a curse on it's target, one that makes them unable to die in any form. It manifests itself during Kenpachi's fight vs Nnoitra, who presumably kills him with a fatal cross slash to the chest (upon Kenpachi reviving, Nnoitra asks him if he was "playing dead")-- The ending text for chapter 309 and the opening text for chapter 310 even questions if Kenpachi was killed or not. Subsequently, the chapter where Kenpachi finally kills Nnoitra (chapter 311) is called "The Undead 4". This works in contrast to "The Undead 2" and "The Undead 3," both chapters telling the tale of Ichigo's and Kenpachi's fight-- Kenpachi is amazed at how Ichigo seemingly brought himself back from the brink of death after he gave him a fatal stab to the chest. Both Ichigo and Nnoitra both ask themselves why is it that they are the ones "doing all of the cutting" but Kenpachi refuses to go down [die/be defeated]-- that might be a part of the "curse" as well. Seemingly Kenpachi "dies" at the hands of Ryod Lloyd posing as Juhabach as well, but recovers yet again off screen.
Wouldn't be a stretch if Unohana cursed Kenpachi that he would revive himself from death like a demon each and every time he crosses the brink of death, with a power boost each time. Even Kenpachi felt himself being killed at her hand, but quickly had to dismiss the thought because it was fucking with his mindset.
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