I did a search involving a username and a keyword. All the results I got had the keyword in the user’s posts. Good enough.
But suppose you’re looking for a thread where you know this person contributed, but you’re not sure they used the keyword. Will your search include threads where they did not use the word, although other people did?
Does that make any sense?
If you enter a string into “username” and a string into “keyword,” my guess is that you’ll only get back threads that cover both strings, rather than just one. So if you searched for “Rilchiam” and “orgasm,” you’d only find threads in which both occurred, instead of one or the other. Of course, within those threads, there may be instances in which the “Rilchiam” and “orgasm” appeared in the same post, and other instances in which they did not.
I think.
If you search for UserX and “word”, you will find threads where “word” appears in a post by UserX. But you will not find threads that include a post by UserX, and “word” appears in a post by UserY.
An easy test: search in ATMB for poster Rilchiam and keyword “strings”, with a “in the last week” date range: you will not find this thread.
You could also use the keyword “coelacanth” to test it.
You’re right. 
Oh well. Thanks, guys!