designer's own words:This hourglass shaped radiator line benefits from the physical “Coanda effect” to enhance the heat transfer. As this effect makes the air to stick to the elements curved surface, the so created pressure difference further enhances the convection, for a better overall efficiency. Also, the main air flow is better directed through the room and not sent to the ceiling. The heat transfer advantage due to this shape has been widely used in the reversed way – in the well known design of industrial cooling towers. While emerged out of technological reasons, the hourglass shape is a visual archetype, as a kind of border between rectangular and round contours. It has always been a designers favourite, the iconic furniture of Warren Plattner being just an example. “Coanda” line of radiators is an example of how an important technological improvement and a remarkable visual effect (try to scroll the "matching design" image!) can be obtained through a slight alteration of the common radiator shape, so very easy to be manufactured.