Major web search engines usually look for documents containing at least one of the words you entered. They try to sort the best matches (by some trade secret scheme) to the top of the list.
The H-Net engine only returns matches in which all your words appear. It still tries to sort them in some manner; you get to choose whether you want them in "best match" order, or by thread or date. Best match isn't well defined here either, but it has to do with the idea that clusters of hits in the same sentence or paragraph are more likely to be good than random scattered hits. More hits is also better, though sheer numbers won't necessarily bring a message to the very top.
The former is a better approach if you're trying to find ANY answer to a question. The latter is better if you're trying to fine tune things to get THE answer, or perhaps ALL answers, to a question.
The former tends to produce thousands of matches, most of which are irrelevant. The latter tends to produce smaller result sets.