No documentation for the beta skin viewer yet.
HT changed the viewpoint navigation to the same keyboard method used in the other editors. Namely A and D keys for left and right, Q and Z for up and down and W and S for zoom in and out. This means that if you want to for instance view the underside of a plane you need to press the Z key while alternately using the mouse to keep the plane in view. Personally I preferred the old way of doing it.
Another change is the save and load view buttons. Once you get the viewpoint to a into a useful spot you can save this position and then load it again whenever you want.
The other change is the lighting options buttons.
"All Lighting" button shows how the skin will look in the game, i.e with the effects of all the bmps combined.
"No Lighting" just shows the base skin without the all the normal, specular etc maps.
"Lighting Only" is everything but the basic skin. This is useful to see how much difference the other maps are making to the skin.
"Specular Only" shows the effect of the specularity map, how the skin reflects the sun's rays.
"Diffuse only" shows how the skin reflects the diffuse light around the aircraft. This seems to be affected mainly by the normal map and the environment map.
"Environment Only" This shows how the skin reflects the ground and sky around it. Naturally this is controlled by the environment map.