I know this is probably already on here, and I've searched the site but haven't found anything specific, I'm mostly just curious to learn its name for future use.
Anyway, this is where the hero/heroine need to learn more about what they're seeking, so they search out the expert (more often than not an embittered old person who has no time for anything at all). Anyway, the expert is dismissive until a keyword/phrase is mentioned, at which point they are immediately cooperative.
I saw this in A Wild Sheep Case
by Haruki Murakami, where the narrator talks to the Sheep Professor, who is highly dismissive of him until he mentions something about a divine sheep with a star on its back. Then he is fully cooperative. So something like that.