An Allen charge is when judges tell juries to continue to deliberate even after the stage that they reach a deadlock, which is when the jurors cannot reach majority on a determination of guilt or innocence. This was decided in a Supreme Court case in 1896. Some individual states will not allow an Allen charge to be instituted due to it being deemed coercive.
Judges will normally tell the jury that it would be emotionally stressful on both sides if the case ended in a deadlock, because it would be more time-consuming and costly. Judges will often note that another jury, which would have to deal with the same facts, would have to be picked.