For anyone who hasn't seen them before, Hidden Search Operators are handy tricks you can use when you're either searching or filtering AO3.
summary: string is a generic way of explaining that you can search AO3 for a specific word that appears in a summary. You can do this from the search bar in the header, from the Any Field box at the top of the Advanced Search form, or from the Search Within Results box at the bottom of the filter menu.
summary: Bruce OR summary: Banner OR summary: Hulk
You need to put quotation marks around your search term if it is more than one word. The quotes make sure that the site searches for those two words together.
The other two operators listed work best in the Search Within Results box.
expected_number_of_chapters: 1 will return results where every fic has only 1 chapter currently posted.
You can use expected_number_of_chapters: -1 if you want results where every fic has more than 1 chapter currently posted.
otp:true will return results where there is only 1 relationship tag on the fic. If you want results where there are 2+ relationship tags (and no fics with only 1 relationship tag) then you can use otp:false
