What is the test for establishing that a spouse has a presumption of undue influence in a property transaction?

Mathews holds that, under Family Code section 721, a rebuttable presumption of undue influence arises when one spouse obtains an advantage over another in an interspousal property transaction, and that the burden of rebutting the presumption by a preponderance of the evidence is on the spouse who acquired the advantage. (In re Marriage of Mathews, supra, 133 Cal.App.4th at pp. 630-631.) On this appeal, the question is not whether the trial court applied the correct standard but, as always, whether the trial court's decision is supported by substantial evidence. (Id. at p. 632; see also In re Marriage of Mix (1975) 14 Cal.3d 604, 614; Howard v. Owens Corning (1999) 72 Cal.App.4th 621, 630-631.)