Hunting is chemical. The flatworm detects and follows the mucus trail a snail leaves, moving along it in the direction of travel. On reaching the snail it extends a tube-like pharynx from the middle of its underside, pushes it into the shell aperture and digests the animal externally before absorbing it.
The spread was partly deliberate. The species was moved to Pacific islands as a biological control agent against the introduced giant African land snail, a pest of crops. It did not solve that problem reliably, and it also consumes native land snails, which on isolated islands are often narrow-range endemics with no defence against a mainland predator. It is now included on lists of the world's worst invasive species.
An honest account has to separate two culprits. Much of the catastrophic decline of Pacific island snails, particularly Partula species, is attributed to the earlier introduction of the predatory rosy wolfsnail. Apportioning extinctions between that snail, this flatworm and habitat loss is genuinely difficult, and estimates differ between assessments.