Principle of Stretching
The principle behind the use of weights and stretching devices is based on the tendency of human tissue to adapt when exposed to sustained tensile force. In other words, when tissue within the body is subjected to consistent, long term stretching it grows to better deal with the stresses placed on it. The cells within the tissue are forced to divide and multiply under the strain of the applied tension, and over time this leads to an increase in tissue mass.
The whole idea of being able to permanently stretch various parts of the human anatomy may seem a little far fetched, but in reality the practice has had quite a successful track record.