What is the test for undue influence in law?

In Verwoord v. Goss, 2014 BCSC 2122, the court commented on when undue influence arises: 208 Undue influence arises in two forms: a) where the disposition was the result of influence wielded for the purpose of bringing about that end, or b) where there is a relationship between a party and the testator (or donee) in which the potential for influence existed... Respecting the former, the burden lies with the party raising the allegation of undue influence on a balance of the probabilities, and requires proof that the testator or donor was "influenced by another person to such a degree that the testator was coerced into doing what that other person wanted, against the will of the testator,"… 209 On the other hand, where an allegation of undue influence is founded on a dominant relationship, and there is evidence establishing that such a relationship existed, the burden shifts to the party in the dominant position with respect to the testator (or donee) to show that this position was not abused in bringing about the transaction... In other words, that party must prove that the relationship with the testator or donor was not abused so as to bring about the disposition in question... 210 This shifted burden effectively amounts to a rebuttable presumption of undue influence... The presumption is properly raised where the disposition is of "sufficient magnitude," and "the potential for domination inheres in the nature of the relationship itself," such as "solicitor and client" and "guardian and ward"... Ultimately, the question is whether the relationship is one which "provides an opportunity for one person to dominate the will of the other"... [citations omitted] [Emphasis Added]

In Wingrove v. Wingrove (1885), 11 P.D. 81, the court said that undue influence requires “to sum it up in a word - coercion...it is only when the will of the person who becomes a testator becomes coerced into doing that which he or she doesn't desire to do that it is undue influence” (p. 82).