I seriously doubt that Pres. Obama actually gave Joe classified information. Possibly Obama declassified them in his mind before giving them to Joe. (It will be fun when he is called to testify on that). Or, more likely, it was disinformation that was used to make it to Hunter and then to his Russian and Chinese employers.
Possibly it was something that caused Putin believe that he can win over Ukraine and provoked him into a hasty action. Like results of Joe’s IQ test. That’s it. That was the secret.