The short answer is that it's a clause designed to save taxes to the greatest extent possible.... you could call it a "heads up" to the executor that s/he needs to make sure s/he uses a competent tax professional who understands both estate taxes and income taxes to make sure a "rookie" mistake doesn't accidentally trigger a boatload of taxes.
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Is this even a large estate? Chances are this provision is meaningless. These provisions are common and placed into estate planning documents, and, in many situations, for people who have no need for them. Speak with a trusts & estates attorney in your area.