vismach.py is a simulator to show 3D previews of a physical machine. it's supposed to be a quick hack and as such doesn't have any documentation. the source is in [lib/python/vismach.py] and the models are in [src/hal/user_comps/vismach]
here are some screenshots:
vismach can be used to verify motions that don't show up in AXIS preview, such as custom kinematics or m-codes
this screenshot shows a "virtual rotary table" using the C-axis
Another example on what one can do with LinuxCNC (ex Enhanced Machine Controller) and Vismach help.