The dramatic impact would be any dramatic moment in a play that further directs the plot. In the final act of the play, the most intense dramatic moment is when Torvald reads Krogstad's letter. This moment not only creates the most emotionally intense moment of the play but it also moves the play towards the resolution. In addition, this moment is significant for the characters because it becomes a moment of revelation for Nora, which further leads to the play's resolution.
When Torvald reads Krogstad's letter and learns of Nora's forgery, he becomes incensed. His only thoughts are of how Nora has ruined his reputation. Nora had expected that he would reciprocate her sacrifice by becoming self-sacrificial himself and take on all the blame for her wrongdoing. However, instead, all he does is question her in anger, "Do you understand what you have done? Answer me? Do you understand what you have done?," which makes Nora realize that he is not the man she thought he was (III). Nora's revelations are further deepened when he continues to call her "a hypocrite, a liar--worse, worse--a criminal!" (III). Because Torvald's reaction is so very different from what she had expected, it makes Nora realize things about both Torvald and herself. She realizes that Torvald has never loved her as she thought he had and never treated her with respect. She also realizes that she is very naive about the world and needs to educate herself. These realizations lead to the play's resolution, which is Nora leaving her husband and children.
Therefore, the dramatic moment of this act is the moment that Torvald reads the letter and reacts to it, and the impact it has on the play is to lead the play to its final resolution. Furthermore, it is significant for the characters because it creates an awakening for both characters.