The purpose of this feature is to ensure that the percent completed towards quotas of a selected group of strata remains even throughout the field. When the smoothing option is active, all strata that have been assigned weights will tend to be filled at the same pace.
When the Smooth check box is activated, weights are adjusted dynamically so that the probability that the next respondent will be picked from a particular stratum is a function of the number of cases remaining to reach the quota.