>>212931459>It's exactly how it works. Below 2.1 your population declines.Population can continue to increase for a while even if TFR is below 2.1 if it was growing beforehand.
That happens because when population is increasing, each subsequent cohort is bigger than the previous. If a big cohort has a below replacement TFR, the newborn cohort might still be larger than earlier cohorts which are dying off.
Let's say there is a population with 3 generations with 100 people in first, 200 in second and 300 people in third generation. Third generation will have TFR of 1.0. We move 1 generation into the future. 1st generation dies (-100 people) and 3rd generation has 150 descendants. Population still increased by 50 people.