In UK divorce law decree nisi is granted when a court is satisfied that the petitioner is entitled to a divorce. Decree nisi does not end the marriage. It is in the nature of a provisional decree. It means that the court is satisfied that the person who issued the divorce petition has proved the contents of the petition and is entitled to a divorce. The decree nisi is a statement by the court to the effect that if there is no reason to the contrary the petitioner will in due course be entiled to apply for decree absolute and thereby dissolve the marriage.
'Nisi' is the Latin word for 'if' and so it makes clear the provsional nature of the decree. It is decree absolute which dissolves a marriage. In practical terms what decree nisi does is to commence the period of six weeks and one day running after which time the petitioner will normally be able to apply for the decree to be made absolute.
No-one usually attends court when decree nisi is pronounced. The spouses are notified in advance of the day upon which it wil be done. In most County Courts this is on a particular day of the week. The District Judge will pronounce decree nisi in public and usually it will be pronounced simultanously in many cases. 'I pronounce decree nisi in cases X, Y and Z' is the usual type of formula.
Occasionally one or both spouses do attend court on this day and if that happens the District Judge will hear that case separately. Usually the reason for such attendance is for the District Judge to hear argument about why one or other spouse should or should not pay any of the costs of the divorce. Sometimes it is because one spouse or another has changed their mind and either wants to withdraw the divorce petition or else to contest something that was previously agreed.
The reason why there is a compulsory delay of six weeks and one day before the petitioner can apply for decree absolute is to allow time for any objection to be put forward as to why the divorce should not proceed. There is an official called the Queen's Proctor who has the opportunity to intervene during this six week period. Such interventions are extremely rare but they do occur from time to time if, for example, it comes to light that the petitioner has forged the respondent's signature so that the respondent has not really had notice of the proceedings.
In exceptional circumstances the court can shorten the time before which the petitioner can apply for decree absolute. This requires a specific application to the court and a hearing before a judge. Typically the reason for such an application is to enable a child who would otherwise be born outside of marriage to be born inside marriage by enabling the petitioneror respondent to remarry within the six week period. Such applications are very uncommon.
Although decree nisi does not end the marriage it does have one important effect. The court does not have jurisdiction to make a final order in respect of financial issues arising from the marriage until at least decree nisi has been pronounced. Therefore you need decree nisi in order to settle ancillary relief proceedings once for all by way of a court order. It is difficult to finalise the financial issues arising from the marriage without decree nisi.