Hull stiffness is related to the pressure in the main chambers.
The floor has the "drop-stitch" construction and this is the key to the stiff / flat floor. This technology has been used to make inflatable surfboards, so it is quite possible to get a solid structure this way. Imagine inflating and object... it wants to be round, so to prevent the floor surfaces from bulging, there are thousands of fibers attached to the two surfaces. These are all at specific length to keep the surfaces at a constant distance from each other. When inflated to pressure, the floor becomes rigid.
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