The second stage of initiation in Dauntless is a lot like the aptitude test: it involves a chair and some mental exploration. It's like a mix of therapy and the movie The Cell (not recommended).
This simulation is all about facing one's fears and how the only way to beat your fears is to not be afraid of them anymore. Luckily, it's conducted privately, with just Four and Tris, so she doesn't have to face an audience.
The simulation also involves another simulation serum, but this time it comes with a transmitter injected into her, which has never ever ended well for any character ever.
Once it starts working, Tris goes into her fear hallucination. First, she's in an empty field and gets attacked by crows.
And that's as far as today's hallucination goes. Tris feels like she freaked out and failed, but Four tells her that she got out in nearly record time. (The simulation ends when you get your fear response under control.)
Four and Tris discuss fear, which is a very mature discussion like you'd see on PBS, except for the fact that Tris has a transmitter in her.
Their chat reveals that Tris isn't really afraid of crows—they're just a "symbol" of something.
And then they discuss Dauntless—how it used to be about courage, but now the leaders are into cruelty. Which is why Eric is in charge of the new initiates, even though Four was ranked above him when they were initiates.