Carl is using a defense mechanism known as displacement, which is the transfer of an emotion from its original focus to another object, person, or situation.

Question

Why do I need a comma after "displacement"?

Answer 1

http://grammar.ccc.commnet.edu/grammar/commas.htm

Read and study #4 here.

Answer 2

I'm a bit confused. Is there always a comma before "which"? Or does it only applies to some?