What is the effect of the presumption of undue influence on a jury?

In his view the presumption of undue influence is a presumption of law, the influence of which on the ultimate issue is limited to the burden of proof. At 397 he said: It must be kept in mind that the peculiar effect of a presumption "of law" (that is, the real presumption) is merely to invoke a rule of law compelling the jury to reach the conclusion in the absence of evidence to the contrary from the opponent. If the opponent does offer evidence to the contrary (sufficient to satisfy the judge's requirement of some evidence), the presumption disappears as a rule of law, and the case is in the jury's hands free from any rule. [Emphasis in original.] and at 398: It is the legal burden or burden of non-persuasion that is occasionally misapplied by treating it as a make-weight in the assessment of the evidence. This is characteristic of some of the undue influence cases that refer to matters that must be proved in order to discharge the burden of proof imposed by the presumption. See Allcard v. Skinner (1887), 36 Ch.D. 145, at p.181.