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Voir dire - Wikipedia, the free encyclopedia
It also refers to the process by which expert witnesses are questioned about their backgrounds and qualifications before being allowed to present their opinion testimony in court. As defined by Gordon P. Cleary: "Voir Dire is the process by which attorneys select, or perhaps more appropriately reject, certain jurors to hear a case."[4] As noted above, in the United States (especially in practice under the Federal Rules of Evidence),
voir dire can also refer to examination of the background of a witness to assess their qualification or fitness to give testimony on a given subject.[5]
The bold part. Many times when they are offering a piece of evidence to be admitted to the record, the other side gets to voir dire the witness as to their knowledge of the item.
I'm no lawyer, so maybe a lawyer can explain it better. I've seen it done in many cases.