(1) As used in this section:

(a) "Advanced fraction" means a fraction that has as its numerator the amount of the advanced tax and as its denominator the value of the interests in insulated property to which that tax is attributable.

(b) "Advanced tax" means the aggregate amount of estate tax attributable to interests in insulated property which is required to be advanced by uninsulated holders under subsection (3) of this section.

(c) "Insulated property" means property subject to a time-limited interest which is included in the apportionable estate and is unavailable for payment of an estate tax because of impossibility or impracticability. Insulated property does not include property from which the beneficial holder has the unilateral right to cause distribution to himself or herself.

(d) "Uninsulated holder" means a person who has an interest in uninsulated property.

(e) "Uninsulated property" means property included in the apportionable estate other than insulated property.

(2) If an estate tax is to be advanced pursuant to subsection (3) of this section by persons holding interests in uninsulated property subject to a time-limited interest other than property to which RCW 83.110A.060 applies, the tax must be advanced, without further apportionment, from the principal of the uninsulated property.

(3) Subject to RCW 83.110A.080 (2) and (4), an estate tax attributable to interests in insulated property must be advanced ratably by uninsulated holders.

(4) A court having jurisdiction to determine the apportionment of an estate tax may require a beneficiary of an interest in insulated property to pay all or part of the estate tax otherwise apportioned to the interest if the court finds that it would be substantially more equitable for that beneficiary to bear the tax liability personally than for that part of the tax to be advanced by uninsulated holders.

(5) Upon payment by an uninsulated holder of estate tax required to be advanced, a court may require the beneficiary of an interest in insulated property to provide a bond or other security, including a recordable lien on the property of the beneficiary, for repayment of the advanced tax.

(6) When a distribution of insulated property is made, each uninsulated holder may recover from the distributee a ratable portion of the advanced fraction of the property distributed. To the extent that undistributed insulated property ceases to be insulated, each uninsulated holder may recover from the property a ratable portion of the advanced fraction of the total undistributed property.