[Greetings_Puny_Humans]

What is more likely, IMO, is that Putin showed Poroshenko a documentary movie that was filmed in last couple of weeks in the theater of war. And then he showed him an invitation to Hague

What a joke! I do not think it is impossible for Putin the mass murderer and thief to be so cynical as to accuse Ukraine of war crimes (they've been doing it since day one), but the idea that Putin has some secret smoking gun is ludicrous. They aren't holding anything back in Russian media, which consists of images of the Ukrainian government with cuts to black and white videos of marching Nazis.

[Greysard]

but the idea that Putin has some secret smoking gun is ludicrous.

It doesn't need to be secret, or even true. It only needs to be effective. In other words, Putin only needs to have some power over Poroshenko. Pick your own kind of power. Any threat will do.