What's the difference between backward(s) and backwardly?

The only real difference that I can tease out, other than frequency trends over the years, is that backwardly can be more easily used comparatively than backward(s), the latter pair being less gradable. For instance, a feature of primate skeletons is the backwardly-facing head of the humerus. Now it doesn't face backwards, it only faces backwards of the sagittal plane. Things that spin can only spin forwards or backwards; so there's no need for gradability. But in a less constrained situation, forwardly movement and backwardly movement means that there is some component of the movement in the indicated direction. Also, software might be touted as backward compatible, but turn out to be less backwardly compatible than had been hoped for.