Section 676(b) provides an exception to the general rule of section 676(a) when the exercise of a power can only affect the beneficial enjoyment of the income of a trust received after the expiration of a period of time which is such that a grantor would not be treated as the owner of that portion, except as power were a reversionary interest. See §§ 1.673(a)-1 and 1.673(b)-1. Thus, for example, a grantor is excepted from the general rule of section 676(a) with respect to ordinary income if exercise of a power to revest corpus in him cannot affect the beneficial enjoyment of the income received within 10 years after the date of transfer of that portion of the trust. It is immaterial for this purpose that the power is vested at the time of the transfer. However, the grantor is subject to the general rule of section 676(a) after the expiration of the period unless the power is relinquished. Thus, in the above example, the grantor may be treated as the owner and be taxed on all income in the eleventh and succeeding years if exercise of the power can affect beneficial enjoyment of income received in those years. If the beginning of the period during which the grantor may revest is postponed, the rules set forth in § 1.673(d)-1 are applicable to determine whether the grantor should be treated as an owner during the period following the postponement.