A couple days ago, a Swiss skier named Andri Ragettli landed the first ever 'Quad Cork 1800', in which he flew 38 yards off of a jump in Italy, making five full rotations and four head-under-body spins. The video of the jump is attached below. The true difficulty of landing such a trick is very clear when considering the physics behind it. First, in order to be in the air long enough to perform such a trick, a skier needs to gain a great amount of kinetic energy as he descends from the top of the hill. In order to do this, the height of the top of the hill should be maximized so as to maximize gravitational potential energy, which is then converted into kinetic energy as the skier descends. Additionally, once Ragettli is in the air, you may notice that he crouches down low, which minimizes the rotational inertia of his body, allowing him to experience a more rapid angular acceleration. After Ragettli rotates multiple times in mid air, just before landing, he straightens his body, which increases his rotational inertia. Since angular momentum is conserved, an increasing rotational inertia causes a decreasing angular speed. Therefore, by straightening his body, his angular speed decreases, making it easier to stick the landing.