What is the test for establishing ineffective assistance of counsel?

discloses that counsel had no rational tactical purpose for [his or her] act or omission.' " [Citation.]' [Citation.] If the record on appeal ' " 'sheds no light on why counsel . . . failed to act in the manner challenged[,] . . . unless counsel was asked for an explanation and failed to provide one, or unless there simply could be no satisfactory explanation,' the claim on appeal must be rejected" ' . . . . [Citation.]" (People v. Vines (2011) 51 Cal.4th 830, 871-872, italics added, overruled on another ground in People v. Hardy (2018) 5 Cal.5th 56, 104-105.)

" 'Judicial scrutiny of counsel's performance must be highly deferential. . . . [I]t is all too easy for a court, examining counsel's defense after it has proved unsuccessful, to conclude that a particular act or omission of counsel was unreasonable. . . . [T]he [appellant] must overcome the presumption that, under the circumstances, the challenged action "might be considered sound trial strategy." [Citation.]' [Citation.]" (In re Valdez (2010) 49 Cal.4th 715, 729-730, italics added.) Where "a valid possible explanation" exists for counsel's alleged failure, the appellant "has failed to establish ineffective assistance of counsel." (Cf. People v. Diaz (1992) 3 Cal.4th 495, 563.)