A type of wall that blocks pressure but allows particles through, the particles retain their velocity. It could be useful for preventing pressure from interfering with pressure sensitive devices.
The reason this doesn't exist is simply because when such a wall exists, elements become hopelessly clogged inside of it due to the way in which velocity and pressure are intertwined. The closest to this we'll likely ever get is Pipe element, which allows elements through but sponges pressure through very slowly.