[Itachi just nods.]

Ganjutsu is the illusionary type of ninja technique. It can make one believe -

[His form suddenly scatters into a mass of crows that go flying in all directions as he continues to speak.]

- that they're seeing things they truly aren't. The Sharingan makes these skills particularly strong in the Uchiha.

[The birds gather back into the seated figure of Itachi, as he hasn't really gone anywhere from the start.]