What is the burden of proving undue influence upon the promisor in a contract?

The onus of proving that undue influence was exercised successfully upon the promisor in such a contract lies upon whoever asserts such: but that onus will shift to the promisee if it is shown that suspicious circumstances exist as to the execution of the instrument under attack: Vout v. Hay (1995), 1995 CanLII 105 (SCC), 2 S.C.R. 876, per Sopinka J. (per curiam) at 889. Earlier that learned Judge stated (p. 888): The suspicious circumstances may be raised by (1) circumstances surrounding the preparation of the will [here, "contract"], (2) circumstances tending to call into question the capacity of the testator [I have dealt with that and found she had the requisite capacity], or (3) circumstances tending to show that the free will of the testator was overborne by acts of coercion or fraud.