Except as provided in this paragraph, it is immaterial in what capacity the power was exercisable by the decedent or by another person or persons in conjunction with the decedent; whether the power was exercisable alone or only in conjunction with another person or persons, whether or not having an adverse interest (unless the transfer was made before June 2, 1924; see paragraph (d) of this section); and at what time or from what source the decedent acquired his power (unless the transfer was made before June 23, 1936; see paragraph (c) of this section). Section 2038 is applicable to any power affecting the time or manner of enjoyment of property or its income, even though the identity of the beneficiary is not affected. For example, section 2038 is applicable to a power reserved by the grantor of a trust to accumulate income or distribute it to A, and to distribute corpus to A, even though the remainder is vested in A or his estate, and no other person has any beneficial interest in the trust. However, only the value of an interest in property subject to a power to which section 2038 applies is included in the decedent's gross estate under section 2038.
For the purpose of the foregoing provision, the term "mental disability" means mental incompetence, in fact, to release the power whether or not there was an adjudication of incompetence. Such provision shall apply even though a guardian could have released the power for the decedent. No interest shall be allowed or paid on any overpayment allowable under section 2038(c) with respect to amounts paid before August 7, 1959.
26 C.F.R. §20.2038-1