She sits like a lady should. She acts as though a lady should act. She does everything someone proper, and of her type should do and be…but he knows she's not. It takes him back to the Titanic, a movie he was forced to watch with his mom every mother's day. It takes him back to the scene where Rose is having lunch with her mother and she sees a little girl being forced to sit up straight and to place a napkin in her lap. It's then that Rose frees herself from the bondage of manners and customs of the family's tricks and trade to success. Where she breaks the overpowering rule and forces of those who've had a hold on her her whole life.
Why this scene goes back to Quinn takes some figuring out. He let the ideas dissolve in his brain before realizing that she is Rose, and he is Jack. Being Leonardo never felt so right to him, he was Jack Dawson, a guy who could free Rose from everything that she let hold her down. Help her realize that there's more to life than what the world perceives as important. Only this time around, he wasn't about to lie in a freezing ocean with Quinn, and he wasn't about to say a goodbye and sink to the bottom. He wouldn't let himself sink to the bottom of her memory.