It seems like a general expression without "is when", but how's the meaning get different including "is when" in the middle of the sentence?
"Is when" is used here to describe an incident that became a turning point in the speaker's life. This expression is used to refer to a specific incident or time. Chandler's parents' divorce is when (the period of time in his life) he began using humor as a defense mechanism. We can think of "is when" as "is the period of time" or "is the time when". Ex: During college is when I started MMA as a hobby. Ex: 9AM is when the grocery opens.
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