The GetFrame_REF_to_abs can return a more complete ChMovingFrame, instead than ChFrame.
This allows accessing accelerations and speeds of the reference frame very easily, because these are automatically updated in ChBody and ChBodyAuxRef (the latter precomputes this for performance, i.e. concatenates the two ChMovingFrames, the COG-to-abs and ref-to-cog, and sets the resulting ref-to-abs moving frame at each Update().)
Use application.DoStep () nenables a better experience of the demo, because the user can stop it (use spacebar) and more important, one can press 'i' to open the 3d view settings, and press the 'draw COG' button , so that the display shows the position of the two COGs of the two pendulums, and also shows the two references (that are different).
A much easier way to get absolute speeds of the REF in the ChBodyAuxRef demo.