I would say it comes shortly after the climax in Act IV, when Henry suggests that Liza essentially prostitute herself by marrying a rich man if she can find no other means of support. Liza has gained more than a large vocabularly; she has gained self-respect. She shoots back at Henry: "I sold flowers. I didn't sell myself. Now you've made a lady of me I'm not fit to sell anything else."
After Liza returns the ring Henry had given her and he throws it into the fireplace, things are winding down, thus the "falling action." Act V will be the actual denouement, with Liza deciding that she will marry, but also teach phoentics, thus asserting her independence but also securing a more comfortable future.