The Hensley hitch does not pivot on the hitch ball as in the usual setup, but through a trapezoidal link system. There are many explanations of the principle on the web such as this one for example. Apparently it is derived from a system used on old tractors and other farm implements.
Below is a video demonstrating the principle of the hitch. It refers to a ProPride hitch which is a Hensley competitor, but the principle is the same.
And then there is this one: