"The Secret of the Impure World Reincarnation" (穢土転生の秘密, Edo Tensei no Himitsu) is episode 264 of the Naruto: Shippūden anime.
Naruto continued his tailed beast training with B. As he attempted to transform into the Nine-Tails, he failed, assuming a chibi-version of the beast. With this, the Eight-Tails explained the mechanism behind transforming into a tailed beast and the cooperation required from both parties to do so. B told Naruto that this might pose a problem as he wouldn't be able to use the Tailed Beast Ball. Moving on with the training nonetheless, B decided to build on the techniques Naruto could perform. Showing them his Rasengan in his Nine-Tails Chakra Mode, B was shocked to see that Naruto's technique was an application of the Tailed Beast Ball. Telling Naruto this, B enquired who taught him that technique and upon hearing that it was created by the Fourth Hokage, Minato Namikaze, B wondered whether this was fate.
At Mountains' Graveyard, Tobi demanded that Kabuto kill Anko Mitarashi. However, Kabuto refused, explaining that he needed to drain Orochimaru's chakra that resided within her so he can increase the binding power on the Summoning: Impure World Reincarnation. Wary of Kabuto's loyalties, Tobi ordered him to demonstrate that technique and all of its secrets, including on how to stop it. Tobi then killed Torune and told Kabuto to use Fū Yamanaka to reincarnate and control Torune. Kabuto complied and elaborated on the technique before taking off to an isolated area. Black Zetsu appears before Tobi and confirms that he planted the White Zetsu's spores on Kabuto and then sets out to do another task for Tobi.