I think this chapter helps shed a little light on what "that jutsu" is. now that we know that Kushina was the former jinchuuriki, it's safe to assume that she was the former Kyuubi jinchuuriki. the Kyuubi recognized her, and seeing how she had special chakra that enables her to subdue the bijuu and she just happened to be in Konoha, all leads to the idea of her being the Kyuubi jinchuuriki. this is where "that jutsu" comes in. before that attack, Kushina had the Kyuubi sealed into her like any other jinchuuriki. the night of attack, Madara used his Mangekyou to take control of the Kyuubi and release it from its seal.
when sealing the Kyuubi, Minato realized that any normal seal will be hacked again by Madara, so the next seal had to be hack-proof to prevent such a disaster. Minato developed the exact same sealing jutsu that Rikudou Sennin used against the Juubi, or developed something close to it. don't ask me how, it just happened. this seal prevents the Kyuubi from breaking out forever. he sealed the Kyuubi into Naruto using his own four elements seal and added Kushina's chakra into it due to its special bijuu restraining qualities. then he left the scroll, which Gerotora stated
was the key to perfecting "that jutsu". in other words, "that jutsu" wasn't complete, and Minato hoped that Naruto would be able to complete it, activate it once he controlled the Kyuubi. it's important to note that the four elements seal is not "that jutsu" and is not the jutsu Rikudou Sennin used. it is just the sealing technique used to store the Kyuubi chakra into Naruto. "that jutsu" is what is used
here. it can only be completed when the Kyuubi is finally under Naruto's control, that is when it is activated. as we saw, the jutsu completely restrained the Kyuubi, who was about to fight back despite loosing his chakra.
although there should have only been the Kyuubi mind left after having its chakra snatched away, "that jutsu" completely vanquished the Kyuubi. "that jutsu", once activated, allows the users chakra to take the form of Rikudou Sennin. you could see the same text markings on Naruto's hands, that were visible when he signed the seal with Gerotora. so we know that the Rikudou Sennin horn stuff was apart of "that jutsu". now, with "that jutsu" perfected, Madara will not be able to take control over the Kyuubi ever again.
if Naruto just took control over the Kyuubi, he would take the chakra of the Kyuubi and block the mind. however, the mind is still there, which gives the Mangekyou something to take over. with the mind sealed away completely thanks to "that jutsu", the weakness has been removed.