Now you may be thinking that this has been discussed countless of times, and that you all you need to know about that jutsu. If that is the case, do please read the last part of the thread in blue color, since I would like opinions on that "prediction". Anyway have fun! 
1. Analysis of "that" jutsu
The first ever mention of "that" jutsu by Jiraya is here:
It's obviously something secret / forbidden. Tbh, I think Kishi originally intended it to be, when Naruto started releasing Kuramas chakra, however, he might have changed his mind later. In general, if you are interested you should read through Jirayas conversation with the key-frog in chapter 370. I will, however, summarize chapter 370 shortly:
I will also use some pages from Jirayas discussion with the key-frog, here for instance is something weird, which suggest it is no ordinary jutsu:
What does the key frog mean, when he says:
"Does Naruto even need that jutsu?"
Also Jiraya says that Minato knew something of grave importance:
I think we can all be sure it has something to do with Obito, and what Obito said, when he fought Minato after pulling the kuybi out of Kushina. Anyway it's probably this:
So fair enough, the key is given to Naruto. No big deal. However, then we go to chapter 489 and 490, where Naruto was last summoned to Mount Myōboku to have his future foretold by the old geezer frog. This is the next time, where "that" jutsu is mentioned:
The key-frog explains a lot about how to gain control over the fox' chakra, and that it is the key to perfecting "that" jutsu.
2. Conclusion:
You need Kuramas yang chakra to complete "that" jutsu.
Whatever it is, it is probably not an ordinary jutsu like rasengan or tbb, since the key-frog wonders, why Naruto would even have need for the jutsu. Though one may wonder if Jiraya would actually have been able to teach Naruto some rare jutsu back then...
Possibly there is a 3rd reason, why people of the Uzumaki clan has been the only jinchurikis through the entire story. People of the senju clan, should be just a suited for it, yet Konoha went as far as to bring Kushina to the village just to become the host.
So it might be that there is a jutsu that only an Uzumaki can use, and it requires perfect control over Kuramas chakra.
Naruto hasn't used it yet.
3. Prediction / Discussion
"That" jutsu is not an ordinary jutsu suited for battle or anything. In particular the fact that the key-frog asks, why Naruto would even need that jutsu, is bothering me a lot. Why would he say that if it was a jutsu intended to fight someone like Obito / Madara?
Minato obviously intended for Naruto to learn "That" jutsu, and what is Kishis reason for keeping it in the dark? I have a feeling that it might be an even bigger surprise, when it is revealed compared to a lot of the other "hot" topics in Naruto at the moment, which is why I think it will be the key to "defeating" the Juubi.
We have already seen several resemblances from the SO6P sealing jutsus to the sealing jutsus used by those of the Uzumaki clan. What if the SO6P left a certain jutsu to this clan, which was supposed to be used to bring back the SO6P in some kind of spiritual form, thus bringing forth a "new" savior? The yang chakra from the kuybi is full of life energy. It even reacts to Narutos surroundings:
The Kuybis yang chakra is full of life-energy that it has the ability to make chakra alive. So what my most far-fetched guess would be, is that the so6p left a tiny bit of his own chakra within all the tailed beasts. These beasts all gave this small part of their chakra to Naruto. Well except for Shukaku of course. Naruto with the full yang-power of the Kuybi could - by using some certain weird jutsu, probably a seal, breathe life into this chakra - either giving him an extreme boost, or actually allowing him to meet the so6p in some spiritual form - which in the end could make him capable of defeating the Juubi.
At least this is my best guess, but yours can be as good as mine, though I think it could be an interesting perspective, if the so6p left some of his chakra in every tailed beast, which is what they gave to Naruto, when he met them.
Sorry for the huge wall of text, and even if most probably didn't read all this, thanks for trying. :ice:
1. Analysis of "that" jutsu
The first ever mention of "that" jutsu by Jiraya is here:
You must be registered for see images
It's obviously something secret / forbidden. Tbh, I think Kishi originally intended it to be, when Naruto started releasing Kuramas chakra, however, he might have changed his mind later. In general, if you are interested you should read through Jirayas conversation with the key-frog in chapter 370. I will, however, summarize chapter 370 shortly:
Summary of chapter 370
Jiraya has a conversation with the frog that holds the key to Narutos seal. He is about to fight Pain, so he tells the frog to store himself with Naruto in case anything goes wrong.
Jiraya says Minato sealed the fox in Naruto for a reason.
He even mentions that it's possible that Madara was behind the 9 tailed fox attack, since only he would be able to summon the fox.
Jiraya has a conversation with the frog that holds the key to Narutos seal. He is about to fight Pain, so he tells the frog to store himself with Naruto in case anything goes wrong.
Jiraya says Minato sealed the fox in Naruto for a reason.
He even mentions that it's possible that Madara was behind the 9 tailed fox attack, since only he would be able to summon the fox.
I will also use some pages from Jirayas discussion with the key-frog, here for instance is something weird, which suggest it is no ordinary jutsu:
You must be registered for see images
What does the key frog mean, when he says:
"Does Naruto even need that jutsu?"
Also Jiraya says that Minato knew something of grave importance:
You must be registered for see images
I think we can all be sure it has something to do with Obito, and what Obito said, when he fought Minato after pulling the kuybi out of Kushina. Anyway it's probably this:
You must be registered for see images
So fair enough, the key is given to Naruto. No big deal. However, then we go to chapter 489 and 490, where Naruto was last summoned to Mount Myōboku to have his future foretold by the old geezer frog. This is the next time, where "that" jutsu is mentioned:
You must be registered for see images
The key-frog explains a lot about how to gain control over the fox' chakra, and that it is the key to perfecting "that" jutsu.
2. Conclusion:
You need Kuramas yang chakra to complete "that" jutsu.
Whatever it is, it is probably not an ordinary jutsu like rasengan or tbb, since the key-frog wonders, why Naruto would even have need for the jutsu. Though one may wonder if Jiraya would actually have been able to teach Naruto some rare jutsu back then...
Possibly there is a 3rd reason, why people of the Uzumaki clan has been the only jinchurikis through the entire story. People of the senju clan, should be just a suited for it, yet Konoha went as far as to bring Kushina to the village just to become the host.
So it might be that there is a jutsu that only an Uzumaki can use, and it requires perfect control over Kuramas chakra.
Naruto hasn't used it yet.
3. Prediction / Discussion
"That" jutsu is not an ordinary jutsu suited for battle or anything. In particular the fact that the key-frog asks, why Naruto would even need that jutsu, is bothering me a lot. Why would he say that if it was a jutsu intended to fight someone like Obito / Madara?
Minato obviously intended for Naruto to learn "That" jutsu, and what is Kishis reason for keeping it in the dark? I have a feeling that it might be an even bigger surprise, when it is revealed compared to a lot of the other "hot" topics in Naruto at the moment, which is why I think it will be the key to "defeating" the Juubi.
We have already seen several resemblances from the SO6P sealing jutsus to the sealing jutsus used by those of the Uzumaki clan. What if the SO6P left a certain jutsu to this clan, which was supposed to be used to bring back the SO6P in some kind of spiritual form, thus bringing forth a "new" savior? The yang chakra from the kuybi is full of life energy. It even reacts to Narutos surroundings:
You must be registered for see images
The Kuybis yang chakra is full of life-energy that it has the ability to make chakra alive. So what my most far-fetched guess would be, is that the so6p left a tiny bit of his own chakra within all the tailed beasts. These beasts all gave this small part of their chakra to Naruto. Well except for Shukaku of course. Naruto with the full yang-power of the Kuybi could - by using some certain weird jutsu, probably a seal, breathe life into this chakra - either giving him an extreme boost, or actually allowing him to meet the so6p in some spiritual form - which in the end could make him capable of defeating the Juubi.
At least this is my best guess, but yours can be as good as mine, though I think it could be an interesting perspective, if the so6p left some of his chakra in every tailed beast, which is what they gave to Naruto, when he met them.
Sorry for the huge wall of text, and even if most probably didn't read all this, thanks for trying. :ice:
