Emission -> Hair Length vs. Velocity -> Normal

I am following a video tutorial about fur and the instructor lowered the Velocity -> Normal parameter to make the fur shorter.

What is the difference between using Emission -> Hair Length vs. using Velocity -> Normal? They both appear to have the same effect on hair, but I suspect there is a nuance I’m missing.

(Blender’s manual doesn’t provide any useful information about Velocity -> Normal as it applies to hair.)