More bone for chakra?? Thats why he has to use his level 2 seal. I would thing this is the only logic for Kimmi's jutsu. If not, I doubt he can extend his bone in outragous porportions. And for 'opening his gates',

I definitely believe the cursed seal is suppose to be like Rock Lee's opening gates technique. He pushes past his own normal limits and maybe destroying himself inside.

Whereas Tsunade's might be borrowing time from the future.

I don't know if Kishimoto thought about it, but suddenly having super chakra from nowhere doesn't make sense. Whereas if Rock Lee's gates and the cursed seal serves more or less the same purpose, that would be more believable or borrowing time.

THen again, everything follows the law of anime.