Board Thread:Questions and Answers/@comment-4620155-20140302032016/@comment-4620155-20140603040705

AvatarReiko wrote: It was most likely his Captain's examination. He probably had to demonstrated all his Zanpatou abilities in front of one of the senior captains, most probably the head captain himself.

I don't think you have to show all your Bankai abilities in the examination - you just need to show that you can perform Bankai. Also, Byakuya might have developed his Senkei technique after the examination.

I'm leaning more towards the thought that the first person to witness Senkei will be revealed as some sort of plot twist.