Patent #'s 10525627 and 10220559
Hollow gaskets can be engineered for selectable compression characteristics. Unlike homogenous foamed gaskets which can be thought of as a spring - gradually applying more force as the seal is compressed - extruded seals can provide initial stiffness and then a sudden decrease in required compressing force.
As a result, it can take surprisingly little force to cause deflection in the seal. This allows a high degree of compliancy in the seal, making this technique ideal for sheet metal sealing applications or allowing sealing against very irregular surfaces.
Alternatively the seal can be designed with a cross section that looks like the letter "B" on its side where the center leg does not get attached to the the bottom. Now, as this seal is compressed, the compressive force is very light initially, but when the center leg bottoms out the force to compress rapidly increases.