The DTM tracks a country through five stages as birth and death rates fall. The gap between the two lines is natural increase. Slide through the stages and watch the rates and total-population curve change.
— birth rate (CBR) — death rate (CDR) ··· total population
Try: step from Stage 1 to Stage 2. Birth rates stay high but the death rate plunges (sanitation, medicine, food) — the lines split wide open and population growth explodes. That early gap, not high births alone, is what launches rapid growth.