Dont use that Jutsu
Manga Chapter 251 page 14
Ive been thinking about it for awhile, and I dont think we've seen what Jiraiya was talking about yet. If you look at the expression on Naruto's face its something that will cause him great harm and or death or as close to death as Naruto will ever get.
1st) Rasengan Shuriken
I think this is incorrect because the 4th never completed the Rasengan he gave up on it, therefor Jiraiya has no way of knowing how damaging it can be to Naruto when hes out of sage mode.
[Manga 321.10] (Kakashi saying the 4th couldn't master the Rasengan)
2nd) 9Tail Fox Chakra
I think this is also incorrect seeing as the use of the Kyuubi's Chakra is not a "jutsu". And as you can see in this [Manga 291.10] when Jiraiya is explaining when naruto is in 4tail mode that he loses all consciousness there for has no clue how much damage he can do to himself or others as evident in the Naruto v Pein arc.
So what do i think it is?
Seeing as Naruto has been trained in all the same ways as the 4th hokage by Jiraiya, Frog summoning, Rasengan(even though the 4th created it), there is only one jutsu that could give Naruto that facial expression, I think its the Fuiin Jutsu - Shiki Fuujin aka the sealing death jutsu the 4th and 3rd used. Seeing as Jiraiya spent the 2.5 years training trying to slowly release the Kyuubi's chakra and alter the seal its possible he taught him this since it was used on him to seal the 9tail fox [Manga 91.12] (here Jiraiya sees Narutos 9tail seal on him for the first time, and shows knowledge about sealing especially unsealing Orochamaru's 5 part seal)
sry cant post links, usually just read forums and not post