Pressing Ctrl-S displays the Search window. When first displayed, search fields are always empty. To retain search criteria between successive searches, uncheck the 'Close window after searching' checkbox at the bottom of the Search window. In this way, the Search window will not close after a search, and search criteria will not be cleared which allows you to reuse this information in successive searches.
When you execute a search, the results of your search appear in the 'Search Results' tab of the Reader window. These results are saved automatically when you quit the program. In a successive search, you can use the previously saved search results as the scope of the next search. In other words, if you specify 'Previous results' as your search scope (near the bottom of the Search window), the next search searches among characters and words in the 'Search Results' tab, not the entire dictionaries. This is a way of fine-tuning, or narrowing, your search.