Here is a simple diagram:
It’s about mtDNA, but works just as well for Y chromosome DNA if you swap the sexes.
Each row is a different generation, so the top row represents the first generation of this population, and the bottom row represents the 5th generation. As you can, this hypothetical population has a constant population size - there are 8 women (men) in each generation. The lines connect ancestors with descendants.
In the latest generation (bottom row), you can see that there are only women (men) coloured in black, indicating (along with the connecting lines) that they descended from the woman (man) coloured in black in the first generation (top row).
If we construct a phylogeny using the 8 modern-day (latest generation) women (men) and then apply Jeanson’s method to it (counting the number of branches through time), we would conclude that the population grew from 1 in the first generation, 2 in the second, 3 in the third, 5 in the fourth, and 8 in the fifth. But we know that the population size was constant, so Jeanson’s method leads to a completely incorrect conclusion and is therefore invalid.