Define your search by expanding the topic-tree and clicking the desired subtopics.
The choosen topics will appear in red.
By clicking "Search highlighted" the query is performed and the results will be ranked by relevance.
The button "Display highlighted" expands the tree up to all your choosen subtopics to make them visible.
The button "Back to Hierarchy" on the results-page restores the shape of the current topic-tree (back to your search).
You can perform a Full Text Search by clicking the Lens Icon above.