I was also thinking the reaper death seal (or some variation to it) as they somewhat refer it like some "Valdemort" jutsu, which could possibly end Naruto's life. But, I don't think Naruto will die just with that tech.
OTOH, it might be an ultimate version of the Rasengan infused with kyuubi's chakra--something that Naruto can only do if he has full control of the kyuubi (and/or in tailed beast mode). Perhaps, Jiraiya forbid that coz Naruto didn't have full control of the kyuubi back then, it could destroy a village (or even a country) or may cause "friendly fire" if it's beyond control and/or it could literally lower his lifespan.
Nevertheless, I'm betting it's a kinjutsu tech.