Science facts about PE. Quite interesting
When the penis grows, there are new cells that are formed. This is called hyperplasia (or hypergenesis).
Quote from wikipedia:
Hyperplasia (or "hypergenesis") is a general term referring to the proliferation of cells within an organ or tissue beyond that which is ordinarily seen (e.g. constantly dividing cells). Hyperplasia may result in the gross enlargement of an organ, the formation of a benign tumor, or may be visible only under a microscope.
As penile exercisers, are goal is to induce hyperplasia within the several structures of the penis. This is where it gets pretty complicated. Due to the different components of the penis, certain cells multiply much slower. Smooth muscle cells multiply very fast, but veins and blood vessels are slow growers.
Not only is hyperplasia important, but hypertrophy also plays an important role when it comes to PE.
Quote from wikipedia:
Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It should be distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number. Although hypertrophy and hyperplasia are two distinct process, they frequently occur together, such as in the case of the hormonally-induced proliferation and enlargement of the cells of the uterus during pregnancy.
Random fact: Megallophalus is basically uncontrolled hypertrophy within the cock.
So, how do we induce hyperplasia and hypertrophy???
Hyperplasia may be due to any number of causes, including increased demand, chronic inflammatory response, hormonal dysfunctions, or compensation for damage or disease elsewhere. Hyperplasia may be harmless and occur on a particular tissue. An example of a normal hyperplastic response would be the growth and multiplication of milk-secreting glandular cells in the breast as a response to pregnancy, thus preparing for future breast feeding.
Hyperplasia may also be induced artificially by injecting hormones such as IGF-1 and human growth hormone. Perhaps the most interesting and potent effect IGF has on the human body is its ability to cause hyperplasia, which is an actual splitting of cells. Hypertrophy, on the other hand, is what occurs during weight training and steroid use and is simply an increase in the size of muscle cells. With IGF use, one is able to cause this hyperplasia which actually increases the number of muscle cells present in the tissue. Weight training with or without anabolic steroid use enables these new cells to mature in size and strength. In addition, animal tests have shown that stretching a muscle can trigger hyperplasia, though this phenomenon has yet to be confirmed in humans. Hyperplasia may also be induced through specific power output training for athletic performance, thus increasing the number of muscle fibers instead of increasing the size of a single fiber. 
PE, jelqing, stretching, hanging, pumping, clamping, squeezing...all this causes our cells to increase in number and in size.
Keep on pulling guys!
reference: Hyperplasia - Wikipedia, the free encyclopedia