Population momentum as a general concept
At the time that Keyfitz introduced the concept, positive momentum (M>1) was the primary concern. However, momentum is equally relevant for populations with negative growth rates. Several high income countries today are characterized by negative momentum (M<1), implying that these populations would continue to shrink even after their fertility rates increased to replacement level. More precisely, negative growth will persist until the relatively large cohorts above the mean age of fertility have died off and mortality falls.
It is also possible, although less commonly done, to describe the population change following a reduction in mortality in terms of momentum. Imagine a population in which there are 10,000 births each year and everyone dies at exact age = 60. Under these circumstances, the total population will be 10,000 × 60 = 600,000. Following an increase in life expectancy where everyone dies at age 70, the population will increase at a rate of 10,000 a year for 10 years. This growth in the population of 100,000 is momentum.