Hahnemann found that often there would be a temporary worsening of the very symptoms he was attempting to cure, so he experimented by diluting the medicine, making it smaller. At a certain point it ceased to act.
Then Hahnemann did something never before tried in the history of medicine. He forcibly shook the medicine every time he diluted it. Holding the vial of diluted medicine in his fist he struck that hand against a heavy book ten or twenty times. This he called succussion. Every dilution was succussed. After diluting and succussing a medicine he noticed that instead of having a weaker effect, it had a stronger one! From then on Hahnemann prepared all medicines by dilution and succussion. They are still prepared in this manner today.
A diluted and succussed medicine is called a potentized medicine. All homeopathic medicines are potentized. Hahnemann noted that potentized medicines acted more forcefully than they did in the gross form, and that the more a medicine was diluted and succussed, the stronger its action. He found that smaller was not weaker but stronger. In fact, medicines became more active the more they were diluted and succussed. Homeopathic medicines are often so dilute that not a single molecule of the original substance is left.