A fall line is also the
path that an un
obstructed object takes down an
inclined plane, or along which an object slides when
suspended from a
catenary, or the path of a
pendulum.
Deviation from the fall line consumes
energy, usually applied as
friction.
For example, skiers who are trying to be badass will take as vertical a fall line as possible down a slope, but when they're going as fast as their nerves can stand they will apply the edges of their skis to push themselves off the fall line and slow down.