What is the test for determining whether a spouse can be found liable for undue influence under section 721 of the California Marriage Act?

(2014) 58 Cal.4th 1396, 1400.) Further, "[i]f one spouse secures an advantage from the transaction, a statutory presumption arises under section 721 that the advantaged spouse exercised undue influence and the transaction will be set aside." (In re Marriage of Mathews (2005) 133 Cal.App.4th 624, 628-629.) The presumption, however, is rebutted if the advantaged spouse can demonstrate that the disadvantaged spouse acted freely and voluntarily, with "'"'"full knowledge of all the facts, and with a complete understanding of the effect of " the transaction.'"' [Citation.]" (Lintz v. Lintz (2014) 222 Cal.App.4th 1346, 1353.)