An "Injured Spouse" should not be confused with an "Innocent Spouse". A taxpayer who has filed a joint return with her spouse and all or part of her share of an overpayment was applied against her spouse's past Federal tax liability, child or spousal support, Federal nontax debt or state income tax may file a claim as an injured spouse to request a refund of the amount of offset. For example, if a spouse is in arrears on his child support, it is possible that any refund generated by the filing of a joint tax return, may be withheld by the IRS and applied toward the arrearage.
A spouse who has income and withholdings or estimated tax payments and who files a joint return with an individual who owes past-due child support must file a request for a refund of her allocation of the joint return refund (Form 8379, Injured Spouse Claim and Allocation).
Documents on IRS.gov
Injured Spouse Claim and Allocation
IRS Form 8379