It's symbolic. The whole thing is like a piece of poetry. Sora, Kairi, and Riku all holding hands on the bech symbolizes their everlasting friendship. Now, Sora doesn't fall through the sand really, it's more like Roxas would be falling from the sky and falls through Sora, but that's a minor technicallity. It symbolizes Sora and Roxas' connection and that's pretty much it. If you can't understand poetry then it sucks to be you because the entire opening is visual poetry.