"Where" is sometimes used to introduce a clause that gives further information about something in a previous clause. It has a number of senses, in this usage: "in which case", "in which condition", "in which circumstances", etc.
You can think of it as a metaphor. Here's the literal use of "where":
1. I went to an island where there were no people, only birds.
And here's the metaphorical:
2. Sometimes she got this look where she pinched her brows together...
Just as "where" in #1 refers back to "island", so "where" in #2 refers back to "look".
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