The following video by Arlan Fuhr, D.C., provides a brief but thorough explanation of exactly how the Activator instrument works. By choosing different tensions on the springs, the doctor can ensure enough force for the sturdy lumbar vertebrae but delicate force for the smaller cervical vertebrae. When the vertebrae subluxate, they quite literally move slightly out of proper alignment with either the vertebra above or below. Choosing the right amount of force ensures that the Activator can nudge the vertebrae back into place.
When using the Activator, it is the doctor's responsibility to maintain a proper angle of use to affect a single vertebrae. Each vertebra has its own tendencies of movement, and its own unique architecture in relation to the neighboring bones, nerves, and muscles. Even an Activator thrust applied to the correct vertebrae but at the wrong angle will likely have no effect. This is why a doctor's knowledge of anatomy is so crucial to gaining your desired results.
Thus, an Activator provides an exact amount of force at an exact amount of speed in a precise location in a precise direction. Phew! So that's how it works.