The front edge of this seat is built on a sprung edge, which is a piece of construction almost nobody sees and everybody feels.
The front edge of a seat takes more punishment than any other part of upholstered furniture. It is where people sit down, where they push up from, and where the weight lands first. Build it simply - stuff the platform and cover it - and that edge gradually collapses. Within a few years the seat has a soft, rolled-under front that no amount of plumping restores, and the crisp line the piece was designed with is gone.
A sprung edge is the traditional answer. A row of springs at the front is tied not down to the frame but to a wire run along the front edge, so the edge itself is suspended and can move. The stuffing is then built over it and stitched into a firm roll along the whole length.
It takes considerably longer to build and it uses more material. What you get is a seat front that still has its shape after a decade of people dropping onto it, and an edge that gives slightly when you sit rather than meeting you like a board.