Okay since alot of people asked in the 418 chapter discussion thread, I'll reveal what I believe "That Jutsu" is. Mind you this is purely theory based on my knowledge of how Kishimoto uses and interprets myths and legends.
First things first, you have to understand a few things before I can fully explain my theory. You need to realize that the names of jutsu, characters and different symbols are directly tied to different legends and myths by Kishimoto-sensei. Kishimoto-sensei also originally intended for Naruto to be the sole main character, however one of his editors suggested that Kishimoto give Naruto a rival, this is how and why Sasuke was created. It is my belief that Kishimoto split the original attributes he was going to give Naruto and divided them between him and Sasuke. So, singular thunder myth, became the Raijin and Fuujin myth. They still however accomplished the same goals. Also, the Fourth Hokage Minato Namikaze was also attributed with thunder god qualities.
With that stated we can move on to the actual theory:
I believe Naruto's "That Jutsu" will be one of two things. Either the VAJRA
or the summoning of the Hindu God INDRA
. I base this belief on how the mythology behind Indra and the Vajra fit into Kishimoto's style along with a few other things that I will explain. Although, please note that Kishimoto is most likely to use the Japanese Buddhism adaptations of Indra and the Vajra.
I've stated this before, but I'll state it again for those that missed it. Naruto's "That Jutsu" is not Fuuton: Rasenshuriken, nor is it Hiraishin no jutsu. Naruto's "That Jutsu" is a completely new technique that Minato Namikaze himself designed and left behind to his son to use for a future threat. Part of the reason Minato sealed the Kyuubi's yang chakra into his own son was, so that his son would have the power needed to perform "That Jutsu". The jutsu requires a tremendous amount of chakra, and the only source that Minato could think of to help power it was the Kyuubi's chakra.
The "that jutsu" that Jiraiya mentions here
, is the same as the one he mentions here
, which is also the jutsu that Naruto himself is practicing here
From the above link about the Vajra:
"Vajra is a Sanskrit word meaning both thunderbolt and diamond. As a material device, the vajra is a short metal weapon that has the symbolic nature of a diamond (it can cut any substance but not be cut itself) and that of the thunderbolt (irresistible force). The vajra is believed to represent firmness of spirit and spiritual power. It is a ritual tool or spiritual implement which is symbolically used by Buddhism, Jainism and Hinduism. Because of its symbolic importance, the vajra spread along with Indian religion and culture to other parts of Asia. It was used as both a weapon and a symbol in India, Nepal, Tibet, Bhutan, Siam, Cambodia, Myanmar, China, Korea and Japan.
The equivalent word in Tibetan is dorje (Wylie: rdo-rje; ZWPY: dojê), which is also a common male name in Tibet and Bhutan. Dorje can also refer to a small sceptre held in the right hand by Tibetan lamas during religious ceremonies."
"In Hindu mythology, Vajra is the weapon of Indra, the Vedic god of rain, lightning and the sky and king of Devas. It is a powerful weapon having the combined features of sword, mace, and spear."
"In Buddhism the vajra is the symbol of Vajrayana, one of the three major branches of Buddhism. Vajrayana is translated as "Thunderbolt Way" or "Diamond Way" and can imply the thunderbolt experience of Buddhist enlightenment or bodhi and also implies indestructibility, just as diamonds are harder than other gemstones."
"The vajra is made up of several parts:
In the center is a sphere which represents Sunyata, the primordial nature of the universe, the underlying unity of all things.
Emerging from the sphere are two eight petalled lotus flowers. One represents the phenomenal world (or in Buddhist terms Samsara), the other represents the noumenal world (or Nirvana). This is one of the fundamental dichotomies which are perceived by the unenlightened.
Arranged equally around the mouth of the lotus are two, four, or eight mythical creatures which are called makaras. These are mythological half-fish, half-crocodile creatures made up of two or more animals, often representing the union of opposites, (or a harmonisation of qualities that transcend our usual experience).
From the mouths of the makaras come tongues which come together in a point.
The five pronged vajra (with four makaras, plus a central prong) is the most commonly seen vajra. There is an elaborate system of correspondences between the five elements of the noumenal side of the vajra, and the phenomenal side. One important correspondence is between the five "poisons" with the five wisdoms. The five poisons are the mental states that obscure the original purity of a being's mind, while the five wisdoms are the five most important aspects of the enlightened mind. Each of the five wisdoms is also associated with a Buddha figure. (see also Five Wisdom Buddhas)"
"Indra's most notable exploit was his battle with the asura Vritra. Vritra took the form of a mighty dragon, and had stolen all the water in the world for himself. No one could do anything about this until Indra was born. Upon hearing what had happened, Indra vowed to take back the life-giving liquid. He rode forth to meet him the terrible Vritra. He consumed great amounts of Soma to give him the strength needed to fight such a foe. Indra smashed through Vritra ninety-nine fortresses, and then came upon the dragon. The two clashed, and after a long battle Indra was able to destroy his powerful enemy. Vritra had been keeping the earth in a drought, but when Indra split open the demon, the waters again fell from the skies. So Indra became a hero to all people, and the gods elected him their king for his victory."
Now that the mythology has been established I'll interpret for you. Indra was a thunder god, just like Raijin and Susano'O(although Susano'O is more of a god of storms). Only two individuals from the Naruto series have direct symbolic relationships to thunder gods. Sasuke Uchiha and Minato Namikaze.
Minato was known as "Konoha's Yellow Flash" and he also created the "Hiraishin no jutsu" or "Flying Thunder God Technique." It's was Minato that originally designed "That Jutsu". However, he was never able to complete that jutsu. He left that task for his son, Naruto. Naruto has now completed(at least close enough) "That Jutsu". It took Naruto becoming a Sage to complete the jutsu. Naruto had to achieve enlightenment in a since to wield "That Jutsu". Indra was also the "god of rain", Naruto's kekkei genkai that I've mentioned before, will give him control over the rain. Why you may ask, well Naruto is a symbolic dragon(So is Pain). They both have the power to control rain, which is the attribute of dragons. For more information on Naruto and his connection to dragons, follow this link.
"That Jutsu" is a combination of the efforts of father and son, it's purpose is most likely to defeat Pain, the other dragon that controls the water and rain. Pain is the symbolic Vritra. Now, I've presented alot of information, but alot of you I'm sure are still doubful about this being "That Jutsu", however Kishimoto-sensei has already left us a hint.
The clan symbol of the Senju Clan of the Forest was a Vajra view from the side.
Naruto 398 page 17
Naruto's "That Jutsu" is a technique that either creates a Vajra or summons Indra himself to wield his "thunderbolt". That is at least the conclusion I've come to. There's still more, but I don't want to make this post longer than it needs to be. If there are any questions I can answer for you, feel free to ask, I'll do my best. Of course, I'm not an expert.