Collapse
In the moment before Korra fell, she looked as if all the gravity of the world had suddenly transferred to her shoulders. Her body was caught in the midst of two polar sensations as her legs gave out. First, the power and the pulse of the Spirit World. Then second, the overwhelming weakness of her body. Or, at least, this is how she explained it to him later, although less eloquently and with more hand motions.
Bolin can't recall every second after her impact in the same way he can remember her blue rage and her collapse after it subsided. He's sure he ran to her, they all did. The only sensation he can recall is panic, but his fears were not as large as hers.
Soon after they reached her, Korra had become cognizant enough to acknowledge the surrounding area, and thus, the destructive power of the Avatar State. She looked at the empty space between him and his sibling and asked the demolished boulevard "What have I done?" He can clearly remember her exact tone of voice, exhausted and peppered with fear. She continued to ask the same question in her shock as the paramedics tried to guide him away. He wasn't aware, until Mako squeezed his shoulder, that aid had arrived, or that Korra was gripping his hand.