Voir Dire is a process to approve a juror or an expert witness for trial. If you don’t think an expert witness knows information that is relevant for your trial, you can question the witness to see if he is not knowledgeable enough and will not confuse or mislead the judge or jury.
Voir Dire for jurors is used primarily to see pick the best jurors for the trial. Someone who will be friendly to your case or your client and is not influenced by bias. As part of the process of selection of jurors, you can exclude up to 6 jurors in misdemeanor cases and 10 jurors in felony cases.
During voir dire attorneys will ask the jurors about their bias. For example, during a DUI trial attorney can ask if anyone was affected by a DUI in the past. When a trial court disallowed questions that would uncover racial biases, the court of appeals reversed the conviction holding that the trial court abused its discretion by not allowing such questions (People v. Wells (149 Cal App 3d 721).