Plotkai means a way for an event happening without explaing itself, like why it happend, and what triggered it. (Mostly used in Anime and Manga series, when a character gains an unexpected powerboost or new abbility just so he can defeat someone stronger than him/her.) These events will be left unexplained nearly allways, for there is no logical explanation for them, exept Plotkai, which means the actual event had to happen to keep the continuity of the story's plot, hence the name: Plot-kai.
Ichigo being able to cut, then defeat Kenpachi is Plotkai, since no other characters have been able to do this, including the ones much stronger than Ichi. He was able to do it because hes the main hero. Thats plotkai. (reference: Bleach)