In the final moments of the second episode of The Sinner, Harry Ambrose (Bill Pullman) makes a startling revelation. While playing Frankie's song -- the same one that incensed Cora (Jessica Biel) to stab her victim to death -- she began pummeling the detective. Then, while examining his bruises, the ever-observant Ambrose made a connection: Cora hit Ambrose with the same pattern of seven hits that she had followed when she stabbed Frankie seven times on the beach.
Here's a zoomed-in look at Cora's stabbing pattern. Why are they the same? Why seven? What is it about that song?