26 C.F.R. § 1.676(a)-1
If a power to revest in the grantor title to any portion of a trust is exercisable by the grantor or a nonadverse party, or both, without the approval or consent of an adverse party, the grantor is treated as the owner of that portion, except as provided in section 676(b) (relating to powers affecting beneficial enjoyment of income only after the expiration of certain periods of time). If the title to a portion of the trust will revest in the grantor upon the exercise of a power by the grantor or a nonadverse party, or both, the grantor is treated as the owner of that portion regardless of whether the power is a power to revoke, to terminate, to alter or amend, or to appoint. See section 671 and §§ 1.671-2 and 1.671-3 for rules for treatment of items of income, deduction, and credit when a person is treated as the owner of all or only a portion of a trust.
26 C.F.R. §1.676(a)-1