The circumcised penis loses sensitivity in three ways:
Loss of the foreskin nerves themselves. As has been demonstrated by studies such as the one by Dr. Taylor and by the testimonials of the majority of intact men, the inner foreskin possesses a greater density of nerve endings. It is thought to be more erogenous than even the glans. The is no question that the foreskin is a highly erogenous tissue. This tremendous amount of sensitivity is lost completely when the forefold of the skin system is amputated. In addition to this, the most sensitive part of the penis, the frenulum of the foreskin, is either partially or totally removed in most infant circumcisions. The frenulum is the continuation of the inner foreskin which attaches to the underside (ventral part) of the glans. Thus, a significant percentage, if not the majority, of erogenous nerve supply to the penis is removed in circumcision at birth.
Damage to the glans. The erogenous sensitivity that remains after circumcision is primarily in the glans. This is further reduced by removal of the protective foreskin which leaves the glans permanently exposed. Unlike the shaft of the penis, and most of the rest of the body, the head of the penis, does not posses its own attached skin. This structure, like the eye ball and the gums of the mouth, is a somewhat naked structure. Its surface is non-keratinized, like that of the gums, the eye ball, and the clitoris in women. That means that it does not posses a protective thick layer like the keratinized skin of the outer penile skin system. Like the gums and the eye ball, the glans of the intact penis has a retractible skin covering. The skin covering of the glans is the foreskin. The eyelid is very similar in architecture to the foreskin. If the eyelid were removed and the eyeball were to become keratinized, you'd have a much harder time seeing. The same is true of the glans. It becomes artificially keratinized (dry, ha rdened, discolored, and wrinkled) as a result of permanent exposure, and thus less sensitive. Because most American men are circumcised and have a glans of this nature, it is harder to notice the abnormality. But just compare the glans of an intact man with that of a circumcised man next to each other and you'll notice a big difference. Thus, in addition to removing lots of erogenous nerve endings in the inner foreskin and frenulum, circumcision further desensitizes the remaining sensitivity of the glans by leaving it exposed.
Loss of skin mobility. The nerve endings in the glans are predominantly complex touch receptors also known as mechanoreceptors. This is different from the light touch receptors of the skin which detect surface friction. The mechanorecptors are best stimulated by massage action rather than surface friction. Thus, the glans is best stimulated to feel pleasure by a rolling massage action. With an ample and highly mobile skin system that rolls over the glans with pressure from the opposing surface, this optimal stimulation of the glans is achieved while avoiding direct friction of the delicate glans surface. Direct friction tends to fire off pain receptors causing irritation and also causes further keratinization of the glans. With the skin system of the penis significantly reduced by circumcision, the mobility is essentially gone and now the penis is a static mass with no dynamic self stimulation mechanism. Now, it must be rubbed. Direct friction is now the primary form of stimulation. So then circumcision further reduces erogenous sensitivity in the penis by reducing skin mobility and thus the ability to use the foreskin to massage the glans. The combination of foreskin and glans in concert results in an even higher level of stimulation which is unknown to the circumcised male.
When you understand what is actually lost to circumcision, it is easy to see how FR helps return this sensitivity to a male. Although there are structures which can never be regrown like the fren and the ridged band, skin expansion can help to repair some of the damage done if some of this sensitive tissue remains and can be expanded. One thing that I have always had trouble understanding is why my whole penis feels as if it has had a sensitivity boost, but now that I have learned about mechanoreceptor nerve cells, I have come to understand that just increased skin mobility is able to make sex more pleasurable.