What is the test for ineffective assistance of counsel?

As explained in People v. Williams (1999) 77 Cal.App.4th 436, 461: "To prevail on a claim of ineffective assistance of counsel, defendant 'must establish not only deficient performance, i.e., representation below an objective standard of reasonableness, but also resultant prejudice. [Citation.] Tactical errors are generally not deemed reversible, and counsel's decisionmaking must be evaluated in the context of the available facts. [Citation.] To the extent the record on appeal fails to disclose why counsel acted or failed to act in the manner challenged, we will affirm the judgment "unless counsel was asked for an explanation and failed to provide one, or unless there simply could be no satisfactory explanation . . . ." [Citation.] Finally, prejudice must be affirmatively proved; the record must demonstrate "a reasonable probability that, but for counsel's unprofessional errors, the result of the proceeding would have been different. A reasonable probability is a probability sufficient to undermine confidence in the outcome." [Citations.]' [Citations.]"