In Tris' landscape what is happening at the cliffs?

In the landscape, as Tris stands near the chasm and waterfall, her mouth is covered by a hand and her body falls into darkness. She describes it as a kidnapping.

I stand next to the chasm. I hear the roar of the water. I scream into the hand that covers my mouth and thrash to free myself, but the arms are too strong; my kidnappers are too strong. The image of myself falling into darkness flashes into my mind, the same image that I now carry with me in my nightmares. I scream again; I scream until my throat hurts and I squeeze hot tears from my eyes.