A blue crab grows by moulting, backing out of its old exoskeleton complete with the linings of its gills and gut. For a short period afterwards the new cuticle is pliable and the animal is defenceless, hiding in vegetation while it takes up water to expand and then mineralises the shell. In warm water the shell can begin to stiffen within hours and is firm again within a day or two.
Harvesters exploit a reliable visual cue. In the days before a moult a pink, then red, line appears along the margin of the paddle-shaped rear swimming leg. Crabs showing the red line are separated into shedding tanks and watched, then removed within a few hours of moulting, before hardening begins.
Mating is tied to the same event. A male carries a female before her final, pubertal moult and mates with her while she is soft, then guards her until her shell hardens. That dependence on a brief vulnerable interval is why soft-shell production is a holding operation rather than a separate fishery.