The Bijuu-mode had the Jinchuuriki and the Bijuu draw chakra from each other. If the Naruto-chakra was drained completely by the Kyuubi, Naruto would die. He was also reminded not to use Kagebunshin in Bijuu-mode, since Kyuubi would immediately draw on all the chakras of the bunshins as well, causing immediate death. The chakra trade was supposed to be negotiated between the two, but Kyuubi didn't seem to be "that type" (figures). Hachibi told Naruto to give up on learning Bijuu-dama (the ultimate chakra blast of a Bijuu).