1.C7 F7 is a "stretch tesuji" that seals the game.

Having a white stone at F7 is key because it immediately threatens 3.F8 F5, which would bring White through via the group in the lower left. (4.A4 A1 falls to 5.A2 D2 and 4.A4 B2 falls to 5.B1 D2.) Having a white stone at C7 creates another formidable threat, namely the follow-ups 3.E9 G10 or 3.E9 F11. There is no way for Black to simultaneously defeat both threats posed by 1.C7 F7. It's worth noting that it does appear Black can ignore everything and just march up the right side with 2.O6 N8; however White can slip past with 3.O4 L4.

In this puzzle, White's first instinct is probably to seize the crucial-looking point E9 but that will not actually work. 1.E9 B9 meets 2.C7 C10 and 1.E9 F7 meets 2.F8 F11!