If your hand was taped in one position for years, and someone cut the tape off and tried to move it, what do you think would happen? Your hand would snap back to the position it was stuck in for all that time. But with repetition - normal movement and alignment can be restored. The same goes for your spine.
When vertebrae are misaligned and stuck chronically over time (subluxated), supporting muscles and ligaments tend to 'hold' them in that position. The first time a Chiropractor tries to adjust it, just like the taped hand, the vertebrae may shift back to the old position it was stuck in. That's why 'one and done' rarely works in Chiropractic. It takes repetition to change.
Frequency and repetition are necessary components of any successful Chiropractic care plan. The more consistent you are with your adjustments, the better the results. For Chiropractors, the definition of sanity is doing the right thing over and over again until health is restored.