One underlying idea is that anxiety or worry competes for working memory, which then diminishes performance.  My impression is that this idea is also linked to how stereotype threat exacts a performance cost.

In studies by Ramirez and Beilock, (combined: 173 students), they found that students who wrote about their thoughts and feelings before an exam, and who rated high in test anxiety, did about 5 percentage points better on the exam when they were in the expressive writing group versus a control group.

For controlled study conditions (laboratory setting), the prompt was “Please take the next 10 minutes to write as openly as possible about your thoughts and feelings regarding the math problems you are about to perform. In your writing, I want you to really let yourself go and explore your emotions and thoughts as you are getting ready to start the second set of math problems. You might relate your current thoughts to the way you have felt during other similar situations at school or in other situations in your life. Please try to be as open as possible as you write about your thoughts at this time. Remember, there will be no identifying information on your essay. None of the experimenters, including me, can link your writing to you. Please start writing.”

In actual exam conditions (ninth grade biology final exam), the prompt was “We would like YOU to take the next 10 minutes to write as openly as possible about your thoughts and feelings regarding the exam you are about to take. In your writing, I want you to really let yourself go and explore your emotions and thoughts as you are getting ready to start the exam. You might relate your current thoughts to the way you have felt during other similar situations at school or in other situations in your life. Please try to be as open as possible as you write about your thoughts at this time.  There will be no identifying information on your essay. None of the teachers can link your writing or any other information to you. If you finish early, please just sit quietly and wait for the teachers’ instructions. You may end up sitting quietly for several minutes while your classmates finish the tasks they were asked to do.  That’s ok. You will be given plenty of time to complete the upcoming exam. This task will only take about 10 minutes in total. Please begin”