Board Thread:Theories and Speculation/@comment-34277398-20180124015008/@comment-1872728-20180201020603

If the target's soul is sealed within the user's soul, that means Minato basically became a jinchuriki before he died. If the target's soul is sealed separate from the user's soul, then I guess Minato fought and sealed Yin Kurama within the Shinigami's stomach.

As for getting the Chakra Mode, either Minato defeated Yin Kurama mentally, told Yin Kurama the reasoning behind the sealing (to fight Tobi) hence befriending him, or Yin Kurama let Minato use his chakra since he knows that Yang Kurama has befriended Naruto.