How is “fold in” used?

In National Theatre Live: Prima Facie (2022), one line is: “She was so composed throughout all her evidence, but afterwards, I saw her fold in two.”

Which movie is “fold in” from?

The clip at the top of this page is from National Theatre Live: Prima Facie (2022).

Phrasal verb

to add one substance to another and gently mix them together

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fold sth in
to add one substance to another and gently mix them together
Fold in the beaten egg whites.

Definiciones del Oxford Advanced Learner's Dictionary, 9.ª edición

She was so composed throughout all her evidence, but afterwards, I saw her fold in two.

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