Herons and egrets have modified neck vertebrae, notably an elongated sixth cervical, that let the neck kink sharply and act as a spring, driving the head forward in a rapid strike. That same structure makes it natural for the neck to fold in flight, producing the hunched profile familiar in a flying grey heron.
Spoonbills, ibises, storks and cranes lack this arrangement. They hunt by other means, tactile sweeping in the spoonbill's case, and fly with the neck straight out and the legs trailing behind, giving a long, level, cross-shaped silhouette. Spoonbills also alternate flapping with short glides.
The rule is a good field mark but not absolute in a careless sense. Storks and cranes fly with necks extended too, so a straight-necked white bird is not automatically a spoonbill; the bill shape settles it. And herons will briefly extend the neck when taking off or landing.