Duck and hoisin is a Chinese restaurant centrepiece reorganised to be eaten in one hand, and the move that defines it is borrowing the pancake idea at sandwich scale. Crispy aromatic duck is normally served as a pile of shredded meat with thin pancakes, hoisin, cucumber, and spring onion to be assembled at the table. The sandwich version takes that same set of components and commits them to a single flexible bread, a wrap or a soft flatbread, so the diner no longer builds each mouthful but carries the whole thing pre-assembled. The hoisin is doing the structural work the pancake handover used to do: it is the sweet, dark, slightly fermented glue that binds rich shredded duck to a neutral wrapper.
The craft is moisture control inside a rolled cylinder. Shredded duck is fatty and the hoisin is wet, so a wrap that is overfilled or over-sauced will blow its seam or go soft along its length before it is finished. The fix is the same one the restaurant uses, only fixed in place: cucumber cut into batons for a cool, water-crisp counter, and spring onion for a sharp green bite, both arranged so each section of the roll is balanced rather than front-loaded. The duck is kept dry and crisp at the edges before it goes in, the hoisin is spread thin so it seasons without soaking, and the bread is warmed so it folds tightly rather than cracking around an uneven load.
The variations stay inside the wrapped-and-sauced frame and mostly argue about the bread and the heat. The flatbread build is sturdier and carries more duck; the soft tortilla version is closer to a deli wrap in feel; a thinned hoisin with chilli pushes it warmer and sharper. Those deserve their own articles rather than being crowded in here.