What is the test for ineffective counsel in a motion for writ of habeas corpus?

"When ... the record sheds no light on why counsel acted or failed to act in the manner challenged, the reviewing court should not speculate as to counsel's reasons .... Because the appellate record ordinarily does not show the reasons for defense counsel's actions or omissions, a claim of ineffective assistance of counsel should generally be made in a petition for writ of habeas corpus, rather than on appeal. [Citation.]" (People v. Diaz, supra, 3 Cal.4th 495, 557-558, 11 Cal.Rptr.2d 353, 834 P.2d 1171.)