From the photo, it appears that the center pulley hinge has two parts. The bottom part sticks into the hollow column, and has a flat bar sticking out away from the camera, and there is a hinge joint on the far end. The top part hinges with the bottom part, and has a shaft supporting the center pulley. It wouldn't take much distance from the hinge joint to the centerline of the pulley to give enough travel to adjust the belt tension.
It is a simple and elegant design, limited only by the need for the hinge to be offset from the centerline of the pulleys. Without the need for belt guards, it works quite well, but if guards are needed, they would have to have a cutout or clearance to avoid interfering with the hinge.