How do HMD and tracker together give 6DoF information

Hi,

I am trying to understand how the sensors on the HMD and the tracker together provide the 6DoF information. More specifically, I wonder if blocking the tracker disables positional tracking totally. 

When I tried that, it seems to give a similar experience to 3DoF with only rotational tracking when I toggle the 'Use position tracking' in Unity. But are they exactly the same? Does the tracker has any contribution to rotation? Also, does the HMD alone provide any translation (considering the head model)?

Thanks,
MC