Same as for the abilities from his Zanpaktou. You get those from meditating and conversing with your sword. Something he only does after getting his ass handed to him and need to do a DBZ esque power-up.
As for Byakuya survivng. I'll put that up to Kubo being weakspined little weakling, would didn't want to deal with butthurt fanboys sending hatemail. Man needs to grow a pair.