A more thorough explanation:

The capture doctrine is a theory in trust law that deals with the distribution of property that has been ineffectively appointed under a general power. According to this doctrine, a power of appointment is considered invalid or ineffective if there is evidence that the person who received the power intended to exercise control over the appointive assets. This is usually demonstrated by the blending of the appointive property with the property owned outright by the donee. The capture doctrine "captures" the property that is the subject of the power and makes it part of the donee's estate. If the invalid appointment is in trust, there will be a resulting trust to the estate of the donee of the power.

In Fiduciary Trust Co. v. Mishou, the Massachusetts Supreme Judicial Court applied the capture doctrine where a testator had completely blended appointive property with any property owned by her outright in her will. The Court found this demonstrated her intent to capture the appointive property for her own estate. The Court explained, "Where the donee of a general power attempts to make an appointment that fails, but the donee has manifested an intent wholly to withdraw the appointive property from the operation of the instrument creating the power for all purposes and not merely for the purposes of the invalid appointment, the attempted appointment will commonly be effective to the extent of causing the appointive property to be taken out of the original instrument and to become part of the estate of the donee of the power."

This doctrine originated in a series of decisions of the English courts and has been applied by a few US states, including Massachusetts, Colorado, and Virginia. The capture doctrine has also been adopted in the Restatement of Property, § 365, and Restatement of Trusts, § 426.

The example illustrates how the capture doctrine works in practice. In this case, the testator blended the appointive property with her own property, which showed her intent to control the appointive assets. As a result, the appointive property was captured and became part of her estate. This doctrine ensures that the appointive property is distributed according to the donee's intent and prevents any unintended consequences.