When you see the bottom of the fish fillets turn opaque, tilt the pan and, using a large spoon, baste the tops of the fish with the hot oil. Do this constantly until the top of the fillets begin to flake, about 3 minutes.
in a recipe. It is unclear to me what it means. For example in https://en.wiktionary.org/wiki/flake closest to this use is "To break or chip off in a flake" what seems to make no sense - especially as recipe explicitly mentions "preventing your fish from falling apart". Example images in recipe also display the fillet in one part, without small parts separated away.
What is the meaning of "begin to flake" in this context?