New Trial Outside the 75-Day Time Limit
In Cobb v. Godfrey, 739 S.W.2d 47, 49 (Tex. Crim. App. 1987), the judge argued that he could grant a motion for new trial outside the 75-day time limit provided in the Rules because of his "plenary power."
The Court rejected this contention, explaining that the former Rule 31(e) required that if a motion for new trial was not ruled on within 75 days, it was overruled by operation of law. Id. at 48-49.
In Awadelkariem v. State, 974 S.W.2d 721, 728 (Tex. Crim. App. 1998) the court held that a judge may "freely rescind" its ruling on a motion for new trial as long as he acts within the 75-day time limit provided by the Rules of Appellate Procedure.
Cobb and Awadelkariem make clear that, to the extent that a judge has plenary power over a proceeding, that power is limited by the Rules of Appellate Procedure and by statute.
Plenary power does not create jurisdiction where none exists under the law; instead, it is a phrase used to describe "a court's full and absolute power over the subject matter and the parties in a case," which only exists as defined by statute or rule.