(a) A conveyance or other contract of a person of unsound mind, but not entirely without understanding, made before the incapacity of the person has been judicially determined, is subject to rescission, as provided in Chapter 2 (commencing with Section 1688) of Title 5 of Part 2 of Division 3. (b) A rebuttable presumption affecting the burden of proof that a person is of unsound mind shall exist for purposes of this section if the person is substantially unable to manage his or her own financial