[Got confused] One needs to make a contract on the froggy scroll before he can.......

[Sage6paths]

Remember Jiraiya requested the froggy (whose body was fused to the contract scroll) to help slightly release the seal on Naruto's belly to release the 9 tails chakra for training.

To make it safe, whoever wanted to unlock that seal needs a contract on a scroll (I assumed).

So how did Orochimaru tighten Naruto's seal on the belly?

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Watch it at 27:57

 

[Punk Hazard]

That was a Fuinjutsu. It affected Naruto's chakra, which in turn affected the seal since they're intertwined.

 

[Zawar]

orochimaru made another seal outside the minato's seal
he didnt messed with the seal

 

[The Necromancer]

Orochimaru added a seal on top of naruto's seal to unbalance it. Once that was removed, THEN jiraiya wanted to looses Naruto's seal. The original 8 pronged seal used to keep the kyuubi inside him.

 

[Sage6paths]

oh i see...

 

[rollin]

it was just another seal

Spoiler

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[pandu]

It was the 5 element seal on top of the eight trigram seal to block naruto's chakra.It has nothing to do with minato's seal.It was a different one altogether.