Air Design developed a new shape that benefits from the advantages of the square shape (pendulum stability, faster opening times, and better sink rates) but without compromising on easiness to fold.
With the Donut shape, you get all of the points above, but also a better ratio between flat and projected surface, which helps a lot on the weight and packing volume, without compromising on sink rate.
This new shape provides fast and smooth openings. It also allows a better ratio between flat and projected surface, resulting in low sink values even with noticeably smaller flat surfaces.
The side vents help make the rescue highly pendulum stable.