When retirement assets are split up in a divorce, it occurs as a result of the Qualified Domestic Relations Order (QDRO) which is a legal order issued by the divorce court. This allows the 401(k) plan sponsor to issue a special non-taxable rollover distribution to the receiving spouse. The receiving spouse can generally then roll the funds into an IRA or a separate account in the same 401(k) plan. Determining the proportion of the plan each party is entitled to is generally part of the divorce mediation or litigation.
Note that you DO NOT want to simply take a distribution of your 401(k) plan and give the funds to your ex-spouse as this would trigger a tax liability that you would be responsible for.
More about a QDRO here: http://divorcesupport.about.com/od/pensionfundsandbenefits/f/qdro.htm
You will need a ‘Qualified Domestic Relations Order’ generally referred to by its acronym ‘QDRO’. It is issued by state courts and is typically part of a divorce decree. Attached is a link to the Department of Labor’s ‘Frequently Asked Questions’ for QDRO’s http://www.dol.gov/ebsa/faqs/faq_qdro.html#.UO3AtGoo6cw