Naruto's "That Jutsu" is a sealing technique with a specific purpose.
I have pieced together, by using both manga and anime to determine what must be Naruto's 'that jutsu' that the J-man taught him.
First, let's start with something interesting.
Naruto's sealing of the kyuubi. When Naruto performed the seal to split kyuubi's chakra from his will, the kyuubi screamed, "What, that's the sage of the sixth paths!" Or something similar. Where the hell did naruto learn a Sot6p level sealing technique? Also interesting to note, with that much chakra, he much resembles the sage of the six paths already.
BUT, that was not "That Jutsu!."
Inevitably, the 8-tails will be captured and sucked into the god statue. And here's the kicker:
So will the 9 tails. Naruto will die. Jyuubi will be reborn.
Kakashi was watching gaara's elder perform the resurrection technique that brought gaara back with his sharingan. Kakashi is a copy ninja. Kakashi can and will resurrect naruto after he loves the kyuubi chakra and dies.
That said, once Naruto is resurrected he will use "that jutsu" to seal the jyuubi within him.
First of all, different jutsu is used for different bijuu sealing into the Jinnchuriki. In Naruto's case, Minato used the Eight Trigrams seal to lock the remaining half of the kyuubi's chakra in Naruto. However, Gaara and Bee and the other Jinnchuriki did not have the eight trigrams seal, they had different ones. Also the Kyuubi only said that it was the so6p because he might have faint memories of when he was apart of the jyuubi sealed within the so6p
And to be honest, i highly, highly doubt Naruto would lose at first and have the nine tails extracted. If this happens, this would mean Minato was wrong in predicting Naruto would utilize the power of the fox to defeat Madara. This would also mean that both Naruto's parents died for nothing in sacrificing themselves to seal the fox in naruto.
I think in the end Naruto will change the nine tails mood and team up with him to use that jutsu
I'm afraid that you have misunderstood what Madara is saying my friend. Madara was merely refering to the jinchuuriki sealing jutsu. In order to become the jinchuuriki of the Juubi, and the very first Jinchuuriki ever, the SO6P invented the Jinchuuriki sealing processs, and ever since then the secrete technique he invented has been passed down and used in different variations to seal bijuu into jinchuuriki. This technique and variants of it have been used ever since by the villages. It was used to seal all the tailed beasts including the Kyuubi which is in Naruto.
This is not the "That Jutsu" Jiraiya spoke about. That Jutsu was invented by Minato for Naruto to complete.
And it will require using the Nine Tails Chakra
So he entrusted The Foxes chakra to Naruto.....hoping he would use the chakra to complete "That Jutsu"
In summary, That Jutsu is a Technique Created or developed for Naruto by Minato, to employ USING THE 9TAILS CHAKRA and to be used against Madara. Naruto already knows how to use the Jutsu, he just hasn't had the means or the reason to do so yet, or until now.
* Now although I have no proof, I suspect that "That Jutsu" will have a Space-Time and /or a Sealing component., because
1) any attack on Madara would need incredible speed or a Space-Time component
2) Minato is a Space-Time expert
3) Uzumaki sealing techniques are a strength of Minato's
** There is another simpler explanation of That Jutsu. Jiraiya's coversation with the key toad could easily imply that Minato wanted Naruto to control the foxes chakra, and this control could be what he meant by "That Jutsu"
The only problem with ur thoery and everyones theroies including mines that what the key frog say this about the jutus(in up manga pages above) "But does naruto have any need for that in the first place" this statement is so puzzling. I dont get it at all, ofc he would need if its a standard kill anyone type of jutsu like madara. DOresnt make sense at all. Im beggging t othink this is a translation error becuase you can think of any sort bs jutsu and you wouldnt say "why do you need it"
Well, a resangan is for killing people, F:RS is for killing monsters, so the Bijuu Bomb (perhaps it's "That Jutsu") would be a technique against semi-gods, or halve immortals, normally, you don't have people like that around even in Naruto world (it's thanks to Kabuto that there are more of them now, so Tobi was at that time, more or less the only one), so a jutsu of that power would useless, when you can still kill everyone with a lesser one (not counting those above monster level).