How check constaint works?

Hi All!I have added check constraint on table. I am calling one function in that check constraint.Actually I have to prevent two records of same type should not have same date range as a result of update. Added check constraint like...ALTER TABLE TABLENAME WITH NOCHECK ADD CONSTRAINT [CHK_TABLENAME] CHECK ((dbo.PAEXTENDADD_DUPLICATECHK (ID, VALIDFROM, VALIDTO) = 1))goJust want to know how does it works?? I mean is it work like first update will be done then go for check constraint validation. if check constraint violated then it will rollback.is it so??Many thanks