CIV § 1229

Where a power to revoke or modify an instrument affecting the title to, or the enjoyment of, an estate in real property, is reserved to the grantor, or given to any other person, a subsequent grant of, or charge upon, the estate, by the person having the power or revocation, in favor of a purchaser or incumbrancer for value, operates as a revocation of the original instrument, to the extent of the power, in favor of such purchaser or incumbrancer.