For a more comprehensive discussion, see Joint Committee on Taxation, Present Law and Background on Federal Tax Provisions Relating to Retirement Savings Incentives, Health and Long-Term Care, and Estate and Gift Taxes (JCX-29-99), June 15, 1999.
The book "Joint Committee on Taxation, Present Law and Background on Federal Tax Provisions Relating to Retirement Savings Incentives, Health and Long-Term Care, and Estate and Gift Taxes" has more information on this.