The skeleton (also known as “Bones” 1) can be displayed for any map. A map can be skeletonized using Calculate -> Map Skeleton.... Use the option menu to choose the map and click “On” then “OK” to the generate the map (the skeleton is off by default).
The level of the skeleton can be changed by using Edit -> Skeleton Parameters... -> Skeletonization Level... and corresponds to the electron density level in the map. By default this value is 1.2 map standard deviations. The amount of map can be changed using Edit -> Skeleton Parameters... -> Skeleton Box Radius...2. The units are in Ångströms, with 40 as the default value.
The skeleton is often recalculated as the screen centre changes - but not always since it can be an irritatingly slow calculation. If you want to force a regeneration of the displayed skeleton, simply centre on an atom (using the middle mouse button) or press the <S> key.