The better question is how Itachi knew the hand seals to make Kabuto release the ET
He probably explored Kabuto's mind like some kind of mind explorer.
He probably explored Kabuto's mind like some kind of mind explorer.
An edo tensei doesn't have complete free will until the moment they are unsummoned/ released by the controller
Itachi used genjutsu to force kabuto to release the edo tensei. Since Madara knows about the jutsu ( either observing tobirama or tobirama telling him about it) he used that brief moment of freedom to use the edo tensei hand signs.
That is why its a forbidden jutsu because an undead is given the opportunity to remain in the world and break free from the casters control.
I remember alot of people moaning how hashirama and tobirama never broke out of edo tensei. It's simple: they were always under oro's control and we're then sealed. Oro never released them from edo tensei so they never had the chance to undo it if they had wished to.
He probably explored Kabuto's mind like some kind of mind explorer.