The power that Krogstad holds over Nora is that he has a IOU that has a signature forged by Nora. This is significant because not only is forging her father's signature a serious crime, but a wife saving her husband's life is seen as an embarrassment in a man's eyes because the husbands in that time were supposed to take care of their wives, not the other way around. If Nora is caught for forging her father's signature, that would bring ridicule and disgrace upon their family and most importantly, on Torvalds reputation. To prevent Krogstad from telling the police, Nora must convince her husband to give him (Krogstad) a raise at the bank so he can regain his reputation back.
