In January 1999, a trust set up by Mitt Romney for his children and grandchildren reaped a 1,000 percent return on the sale of shares in Internet advertising firm DoubleClick Inc.

If Romney had given the cash directly, he could have owed a gift tax at a rate as high as 55 percent. He avoided gift and estate taxes by using a type of generation-skipping trust known to tax planners by the nickname: “I Dig It.”