snap to?

I saw Mrs. Grayson’s spine snap to. She look over her hunsband.
What does the phrase 'snap to'mean in this sentence?
BTW, before the sentence is a warning told by someone else.
A Jewish couple lived in a hotel in South Florida in 1947. At that time the Jews were discriminated against in the South. The hotels in Palm Beach were not supposed to be open to the Jewish people. One day, the manager said he wanted to have a word with the couple after he diiscover their nationality. So follows the above sentence.