You can enable it in "Admin home > Wiki > Structures > Structures Drill Down Menu"
Of course you need to have Structures enabled.
Once enabled, you need to create a structure with several hierarchical levels, and several sister pages in each level. and go to view one children page of the structure.
Then, when the user passes the mouse over a node in the line indicating the path to that node in the structure hierarchy (the sort of breadcrumb line at the page header belonging to a structure), the names of all the children of that node will be displayed below the path to that structure node as links for easier navigation.
Example for the page Structures here in doc.t.o: