Cornu aspersum is a simultaneous hermaphrodite: each animal carries both male and female organs and both partners can be inseminated in a single mating. That arrangement creates conflict, because sperm from a rival can be digested rather than stored.
During courtship each snail everts a dart sac and fires a hardened calcareous dart into the flesh of the other. The dart is coated in mucus containing bioactive compounds that act on the recipient's reproductive tract, reconfiguring the passage so that more of the shooter's sperm reaches storage instead of being destroyed. Snails that land a dart father a measurably greater share of the resulting eggs.
The common misreading is romantic: the dart is often described as stimulating the partner or cementing a bond. It does neither. It is a manipulation, and it carries costs, since a badly aimed dart can pierce internal organs. The dart is also not the delivery mechanism for sperm, which is exchanged separately by ordinary copulation afterwards.