Some people are born double-jointed in which they have tremendous flexibility to the point that they can bend their arms and their legs forty-five degrees backwards. An even more rare occurrence is when a person is born with limbs makes he or she a contortionist. That means that a contortionist can bend their joints backwards a full ninety degrees backwards.
However, this video is absolutely ridiculous. This lady in the video is so flexible that she is able to turn her limbs a full 180 degrees. That looks like it would hurt.