The ending of Catching Fire reveals that the destruction of the arena was a planned rescue by rebels. Katniss wakes in a hovercraft and learns that Peeta and Johanna were taken by the Capitol. She attacks Haymitch in anger and is sedated. In District 13, Gale tells her that her family is safe but that the Capitol has destroyed District 12.
So, what is the twist? In my opinion, it is that the Quarter Quell was never only a contest. The film sets the 75th Games up as a fight between past victors, and the closing minutes show that the arena's destruction was a planned rescue, with Plutarch Heavensbee (Philip Seymour Hoffman) among those who greet Katniss when she wakes. It could be argued that this is why the end of the arena feels so deliberate: a wire run to a single tree that lightning strikes, and a hovercraft that arrives in time.
The sequence is built from the sacrifices that come before it. Mags gives up her life so Finnick can save Peeta, the District 6 victor dies shielding Peeta from the mutated mandrills, and Wiress is killed in the Careers' ambush. Beetee's plan only works because those allies were kept alive long enough to reach the tree. Then Johanna knocks Katniss out and runs off with her tracker, which in hindsight reads as part of the same rescue. Katniss fires her arrow, trailing the wire, into the arena's roof just as the lightning hits, and the force field breaks.
The ending also differs from the first film in a way that matters for the saga. The Hunger Games (2012) closes on a victory, while Catching Fire closes on a loss and a cliffhanger. Katniss escapes, but Peeta is in the Capitol's hands and her home is gone. Cinna's beating by Peacekeepers, just after the wedding gown turns into a Mockingjay, is the earlier sign that the Capitol has stopped treating her as a victor and started treating her as a symbol to be broken.
There is a production angle as well. Gary Ross directed the first film, and Francis Lawrence took over with a $130 million budget, which paid for the Hawaii arena and the finale. The result earned about $870 million worldwide, according to Box Office Mojo, more than the first film took. That is where mockingjay part 1 picks up, with Katniss in District 13 and the rebels asking her to become the Mockingjay.
Perhaps the most useful way to read it is as a bridge film. It resolves the arena, and it hands the open war against the Capitol to the next film. It also sets up the prequel's irony: ballad of songbirds and snakes shows young Snow shaping how the Games work, and here an older Snow uses a rule of the Games against the girl who defied him.