Subject to the exception hereinafter stated, there is hereby levied and imposed upon all generation-skipping transfers occurring after December 31, 1987 that involve property subject to tax pursuant to Section 40-15A-3 hereunder, a tax equal to the full amount of state tax permissible when levied by and paid to the State of Alabama as a credit in computing any federal generation-skipping transfer tax imposed on such transfer according to the act of Congress in effect, on the date of the transfer. The tax hereby imposed shall not exceed in the aggregate amounts which may by any law of the United States be allowed to be credited against such federal generation-skipping transfer tax. The generation-skipping transfer tax hereby levied shall be levied only so long as and during the time a generation-skipping transfer tax is enforced by the United States against Alabama generation-skipping transfers and shall only be exercised or enforced to the extent of absorbing the maximum amount of any deduction or credit which may be permitted by the laws of the United States now existing or hereafter enacted to be claimed by reason thereof as a deduction or credit against such similar tax of the United States applicable to Alabama generation-skipping transfers. The rates of taxation, the method of arriving thereat and the definition of “generation-skipping transfer” and “transferor” shall be as provided in such act of Congress.