What is the test for a motion for a new trial based on ineffective assistance of counsel?

When the trial court has denied a motion for new trial based on an ineffective assistance of counsel claim, we apply a standard of review applicable to mixed questions of law and fact, upholding the trial court's findings if supported by substantial evidence and reviewing de novo the question of whether the established facts demonstrate counsel was constitutionally ineffective. (People v. Taylor (1984) 162 Cal.App.3d 720, 724-725.)

A trial court has "authority to grant a new trial on the ground of inadequate representation of counsel," even though it is not one of the enumerated grounds in the statutory provision ( 1181) for ordering a new trial. (People v. Fosselman (1983) 33 Cal.3d 572, 577-578; People v. Callahan (2004) 124 Cal.App.4th

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198, 209.) To prevail on such a motion, defendant bears the burden to "show that trial counsel failed to act in a manner to be expected of reasonably competent attorneys acting as diligent advocates" and that "counsel's acts or omissions resulted in the withdrawal of a potentially meritorious defense." (People v. Fosselman, supra, 33 Cal.3d at p. 581.) In cases where counsel's acts or omissions do not amount to the withdrawal of a defense, the defendant may alternatively show "that it is reasonably probable a determination more favorable to the defendant would have resulted in the absence of counsel's failings." (Id. at p. 584.) When evaluating a trial court's decision on a defendant's motion for new trial, "[r]eviewing courts will reverse convictions on the ground of inadequate counsel only if the record on appeal affirmatively discloses that counsel had no rational tactical purpose for his act or omission." (Id. at p. 581.) "In all other cases, the conviction will be affirmed and the defendant relegated to habeas corpus proceedings at which evidence dehors the record may be taken to determine the basis, if any, for counsel's conduct or omission." (Id. at pp. 581-582.)