California law presumes a defendant is competent and places on the defendant the burden of demonstrating otherwise. ( 1369, subd. (f).) A defendant is mentally incompetent if, "as a result of mental disorder or developmental disability, the defendant is unable to understand the nature of the criminal proceedings or to assist counsel in the conduct of a defense in a rational manner." ( 1367, subd. (a).) The trial or conviction of a mentally incompetent defendant violates the due process clause of the Fourteenth Amendment to the United States Constitution and article I, section 15 of the California Constitution. (People v. Hayes (1999) 21 Cal.4th 1211, 1281 (Hayes).) In addition, section 1367, subdivision (a) provides: "A person cannot . . . have his or her probation . . . revoked while that person is mentally incompetent." Thus, these statutory and constitutional provisions apply to probation revocation proceedings such as the one before us. (People v. Hays (1976) 54 Cal.App.3d 755, 759.) "Under both the federal Constitution and state law, the trial court must suspend criminal proceedings and conduct a competency hearing if presented with substantial evidence that the defendant is incompetent. . Substantial evidence of incompetence exists when a qualified mental health expert who has examined the defendant states under oath, and 'with particularity,' a professional opinion that because of mental illness, the defendant is incapable of understanding the purpose or nature of the criminal proceedings against him, or of cooperating with counsel." (People v. Mai (2013) 57 Cal.4th 986, 1032-1033 (Mai).) "Defense counsel's expressed belief that defendant might be mentally incompetent does not automatically trigger a section 1369 competency trial." (People v. Sattiewhite (2014) 59 Cal.4th 446, 465 (Sattiewhite).) While counsel's assertion of a belief in his client's incompetence is given "some weight," it is insufficient on its own to require the court to hold a competency hearing. (Mai, supra, 57 Cal.4th at p. 1033 "counsel's assertion that his or her client is or may be incompetent does not, in the absence of substantial evidence to that effect, require the court to hold a competency hearing".) To trigger the court's obligation to hold a competency hearing, "defense counsel must present expert opinion from a qualified and informed mental health expert, stating under oath and with particularity that the defendant is incompetent, or counsel must make some other substantial showing of incompetence that supplements and supports counsel's own opinion. Only then does the trial court have a nondiscretionary obligation to suspend proceedings and hold a competency trial." (Sattiewhite, supra, at p. 465.) Although a defendant's demeanor and irrational behavior may constitute substantial evidence of incompetence, "disruptive conduct and courtroom outbursts by the defendant do not necessarily demonstrate a present inability to understand the proceedings or assist in the defense." (Mai, supra, at p. 1033.) "Absent a showing of 'incompetence' that is 'substantial' as a matter of law, the trial judge's decision not to order a competency hearing is entitled to great deference, because the trial court is in the best position to observe the defendant during trial. . '"An appellate court is in no position to appraise a defendant's conduct in the trial court as indicating insanity, a calculated attempt to feign insanity and delay the proceedings, or sheer temper." .' ." (Mai, supra, 57 Cal.4th at p. 1033.) Consequently, we may not overturn the trial court's refusal to hold a competency hearing unless we conclude either that the record raises a reasonable doubt as to the defendant's mental competence as a matter of law or that the trial court has abused its discretion. (People v. Welch (1999) 20 Cal.4th 701, 740 (Welch).)