Counterillumination is the base mechanism. Photophores on the belly emit a soft green light that matches downwelling light, hiding the animal from anything looking up. The cookiecutter's unlit throat collar breaks that camouflage in one small place, and the resulting dark shape against a glowing background resembles a small fish silhouette.
What happens next is the point. A predator approaching that shape encounters a shark with suctorial lips and a saw of enormous, joined lower teeth. It attaches, bites, and rotates, removing a round plug of flesh roughly five centimetres across. The wounds are so distinctive that they are recognised on tuna, dolphins, whales, seals and even on submarine sonar domes and undersea cables.
The lure hypothesis is plausible but not proven. Nobody has watched a predator approach the dark collar and be bitten; the evidence is anatomical and inferential, and other explanations for the unlit band have not been ruled out.