What the statute requires, in its own words. Section 2056(b)(7)(B)(i) defines qualified terminable interest property as property that passes from the decedent, in which the surviving spouse has a qualifying income interest for life, and to which the election applies. Clause (ii) then defines the income interest: the surviving spouse must be "entitled to all the income from the property, payable annually or at more frequent intervals," and no person may have a power to appoint any part of the property to anyone other than the spouse. That second condition has a carve-out that makes the whole structure work, and it sits in the sentence immediately after: it "shall not apply to a power exercisable only at or after the death of the surviving spouse." So a remainder fixed for the children is fine; a trustee's power to hand principal to them while the spouse is alive is not.
This is the answer to why a trust needs section 2056(b)(7) at all. Left to the general rule, an income interest for the spouse with a remainder to someone else is exactly the terminable interest section 2056(b)(1) disallows.
The election is the whole mechanism, and it is one-way. Section 2056(b)(7)(B)(v) says the election "shall be made by the executor on the return of tax imposed by section 2001. Such an election, once made, shall be irrevocable." Two practical consequences follow. First, the deduction depends on somebody filing a return and ticking a box, which is a different kind of risk from a deduction that applies automatically. Second, the election can be partial: clause (iv) provides that "a specific portion of property shall be treated as separate property," so an executor can elect on a fraction of a trust and leave the rest to be sheltered by the deceased spouse's own exclusion. That partial election is one of the main post-death planning levers a family still has after a first death.
The bill arrives at the second death, and it does not fall where people expect. Section 2044 includes in the surviving spouse's gross estate the value of any property in which the spouse had a qualifying income interest for life and for which the deduction was previously allowed, and section 2044(c) treats it as passing from that spouse. So the trust's full value at the second death, growth included, is taxed in the estate of a person who never owned it and whose own children may receive none of it. Congress anticipated the unfairness: section 2207A(a)(1) entitles the surviving spouse's estate to recover from the people receiving the property the difference between the tax actually paid and the tax that would have been payable without it. Section 2207A(a)(2) lets the surviving spouse waive that recovery by saying so specifically in a will or revocable trust, and a waiver made carelessly shifts a large tax from the first spouse's children onto the survivor's own.
Two edges worth knowing. Section 2519 provides that any disposition of all or part of a qualifying income interest is treated as a transfer of all the other interests in the property. A surviving spouse who gives away or sells the income interest therefore makes a taxable gift of the entire remainder, not of the modest interest actually transferred, and section 2207A(b) gives a matching right of recovery for the gift tax. Separately, because the property is in the surviving spouse's gross estate, section 1014(b)(10) gives it a new basis at that second death, so the remainder beneficiaries inherit it revalued rather than carrying the first spouse's basis forward.