This screen ought to be made more visible (ideally there should be
Advanced Search written at the top right on every page next to the search bar & button) because I honestly have never encountered this page all this time I was here.
Currently the only way you can access that page is if you type nothing in the search bar (which is not user friendly) or if you bookmark and go to this obscure URL:
https://bitcointalk.org/index.php?action=search;It is Search tab at the top on the right side.
If you open a topic, and click on Search box at the top on the left side, and type keyword to search; you will get results inside that topic only. This search feature was revealed by theymos.
note that if you use the search box in the upper right while in a topic, it searches only that topic.