Rules 1.2 and 1.3 of The Bluebook (21st ed. 2020) covers the use of signals. Signals are used in front of citations to alert the reader as to how that citation is being used. They can be quite confusing!
Do not use a signal with a citation when you have directly borrowed someone else’s words or have paraphrased her words or ideas.
Signals can also be used as verbs in text.