Sort is a blocking operator. Don’t feel bad if you haven’t heard of the term; I’ve been working with SQL Server for 15 years, and I’m sure I never heard the term until the incomparable Grant Fritchley mentioned it while he was lecturing at my place of employment.
So sorts and several other types of operators (eager spools, remote query\scan\etc, hash match joins, and more) will block the normal flow and gather all their results before passing any rows on. The hash match join only blocks while building its hash table from the first input, before probing the second.
Read the whole thing. Jared is just getting started with blogging, too, so go pay his blog a visit.