The rarely granted intervention permits the judge to exercise discretion to avoid extreme and unreasonable jury decisions.
[1] A judge may not enter a JNOV of "guilty" following a jury acquittal in United States criminal cases.
If the judge grants a motion to set aside judgment after the jury convicts, however, the action may be reversed on appeal by the prosecution.
[2] A JNOV is appropriate only if the judge determines that no reasonable jury could have reached the given verdict as a matter of law.
For example, if a party enters no evidence on an essential element of their case but the jury, the finder of fact, still finds in their favor, the court may rule that no reasonable jury would have disregarded the lack of evidence on that key point and reform the judgment.