Naruto says "if you promise in name of hokage then I will tell you dattebayo" hokage says "ok" thinking what he can ask is year worth ramen from Ichiraku. Naruto says let me learn one jutsu of hokage cause in academy every one has their family secret jutsu only i don't have one whole making puppy eye. hokage said ony one but I will give you option cause there are many jutsu which will kill you . Naruto said I have very resilient body and very high chakra so give me difficult one like s class or something. hokage said
1)flying raijin it can teleport you where you marked your sign greatly used by fourth hokage 2) shadow clone jutsu but it uses continuous chakra so it is deadly if you don't know your chakra reserve, but I think it can be improved second hokage who created it also said that 3) summoning jutsu it summons helper like monkey king you see here I just summoned and it depends on person to person what you summon. 4) rashomon it is very powerful defense jutsu used by first hokage 5) rasengan created by fourth hokage but it's incomplete and jiraya one of legendry sanin only knows it now 6) sage jutsu I can only give you scroll cause I don't know how to master it but if mastered it's most powerful technique first used by first hokage now used by jiraya.
chose between this all others are too dangerous for you now. Naruto said shadow clone jutsu I will chose and he learns it says thank you old man and goes near the fountain to practice it.he seats there think and says to kurama hey kurama. Kurama replies I have way to make my chakra reserve big but it will use you life power but it can be restored when I learn sage jutsu from jiraya Sensei but it will be painfull for both of us but will increase both our capacity to hold chakra permanent. it's like this I will unlock the seal you will take control over my body and try to expand my chakra reserve from inside cause you live there and I will do same for you but you have to constantly heal both of us using your and mine energy mixing together to form fused healing power but will use only tiny bit of both of our chakra this will cause immense pain but will greatly increase chakra reserve permanently like it will double our chakra reserve. do you wanna try it .
Naruto said both of us had died once nothing to fear let's go and does it after 9 to 10 try they succeeded and Naruto chakra reserve increased two times and kurama's 1.2 times they did that 2 more times and now Naruto chakra reseve was 6 time bigger than original and kurama was 2 times after this . Kurama said let's never try this cause it was forbidden Jutsu created by a very powerful ninja in past and he died using it 4 time. We have used it more 30 times but after this it will become very life threatening as you know ,bnow we have used it too much time now we have to strengthen our chakra reserve's thin boundry or it will break.
Naruto said best way to do it is first let the body rest and make the chakra flow very slow so it will sediment on chakra reserve to thicken it. and then practice extreme tai jutsu to not let body get damaged by use of chakra and Naruto says hey I am gonna seal you now so that people will not get suspicious of us. And sleeps midway talking due to exhaustion.