You convert a fixed problem ('locate search terms that have a high correlation to what I am searching for') to a fuzzy problem of "how can I guess what this person is searching for, based on a sequence of search terms, where I manipulate the results, thereby gauranteeing that the search sequence is poisoned by what I presume they are searching for').
Oddly enough, the latter, is how you get high dwell on a search engine (COUGH COUGH).
as soon as I started using duckduckgo - bang, back to 1 minute searches as I just altered my search terms and actually got back, what I wanted.