Kettlebells aren't ordinary weights. A weight represents an extraneous load on the muscles, joints, ligaments and tendons and it forces the body to go through a range of motion that moves its own weight plus the additional load you've picked up. This increases the body's mass. Weight is the result of that mass being pulled to the centre of the Earth by the planet's gravity. Moving the weight requires us to resist that pull, hence weight training is often called resistance work. But why are kettlebells special? Because the additional movement we apply, the jerks and swings, also add centrifugal force which means our muscles experience the load of centripedal force. Cook stuff if you did geometry at school but in plain English: basic movements become harder and we become stronger as a result.