The chiropractic activator is a small handheld spring-loaded instrument which delivers a small impulse to the spine. It was found to give off no more than 0.3 J of kinetic energy in a 3-millisecond pulse. The aim is to produce enough force to move the vertebrae but not enough to cause injury. A few studies suggest that the activator may be as effective as manual adjustment in treatment of back pain.
(source - Wikipedia)
Activator is often used in cases where the patient has sensitvity issues such as with fibromyalgia, in cases of severe injury/degeneration, or by patient preference for this method.