[quigonkenny]

^This. Ichigo died and his hollow part took over. Period. End of story. There's no rhyme or reason to it (other than him having a hollow part that could take him over) because Kubo had to give Ichigo a massive boost in order to be able to take out Goth Jesus.

That's how true advancement occurs in Bleach. When your ass is kicked long enough, you mystically pull a new (or for some reason previously unused) ability out of said ass and use it to get over (or to trigger your foe's next asspull). Note that if the ability is successful, it is instantly nerfed, and will probably never be more than marginally useful again.

The hole in the chest was symbolic. It was Kubo's ham-handed attempt at foreshadowing.