A post nuptial agreement is a written document agreed to, and signed by a couple after marriage.
The agreement could outline the division of assets, finances, child custody and other matters in the event of a separation or divorce of the couple.
Post nuptial agreements can be enforced by the court. Effect would be given to the agreement unless compelling reasons to the contrary are shown. It is in the interest of both parties to seek legal advice before signing such an agreement. The agreement could be unenforceable if it is proven that it was signed under duress, coercion etc.

It is not every type of duress or coercion which is suffice to nullify the consent of a party to a contract and thereby nullify the contract. It is only actual or threatened physical violence to, or unlawful constraint of, the person of the contracting party which would suffice for the purpose. In determining whether there has been coercion, the complaining party has to show that he had no other reasonable alternative.

The effect of the signed agreement is that the parties who are of full age and understanding are bound by the terms.
If a couple is contemplating divorce, it is sometimes in the interest of both parties to settle their issues through an agreement rather than going through a lengthy trial. Settling the issues saves time and cost.