During the climax of the novel, Emma realizes that she’s in love with Knightley, right after Harriet reveals that she is in love with Knightley and believes him to love her in return. As Emma’s thoughts swirl, she uses a metaphor to describe her experience:
A few minutes were sufficient for making her acquainted with her own heart. A mind like hers, once opening to suspicion, made rapid progress; she touched, she admitted, she acknowledged the whole truth. Why was it so much worse that Harriet should be in love with Mr. Knightley than with Frank Churchill? Why was the evil so dreadfully increased by Harriet’s having some hope of a return? It darted through her with the speed of an arrow that Mr. Knightley must marry no one but herself!
The metaphor—“it darted through her with the speed of an arrow” (“it” being the revelation of her love for Knightley)—captures the intensity and urgency of this realization. In capturing this urgency, the metaphor also highlights how intense Emma’s romantic feelings for Knightley are in comparison to her previous “feelings” for Frank, which were always tempered and lackluster. Ultimately, this moment is evidence of how Emma not only misjudges other people but also misperceives (and represses) her own feelings.