What causes an erection? or.. "the mechanics of a hard-on"
When you get aroused, your brain releases a hormone that causes your body to send more blood to your penis, which fills the corpora cavernosa.
Simply put, the hormones cause the arteries which carry blood to the penis to dilate, or get larger, while causing the veins which carry the blood out of the penis to constrict. So there is more blood flowing in that there is flowing out.
This causes the tissue in the penis to fill with blood and causes an erection!
The technical description is that impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the spaces.
The blood creates pressure in the corpora cavernosa, making the penis expand. The tunica albuginea helps trap the blood in the corpora cavernosa, thereby sustaining erection.
Maintaining effecient blood flow within the penis helps the length increase, the girth widen, the erection last longer, and remain firmer. In much the same way as a swimmer's lungs are more efficient and can hold more air longer, the same is true with your penis.
By increasing the blood flow, the full potential of the erectile tissue is achieved, there by maximizing the penile lenth, and the erection.
Basic math tells you Length x Width = Area.
With a longer shaft and more girth it has, the surface area of the penis is now increased. So now, the nerves thoughout the skin covering the penis are more able to send ALL the sensations back to the brain.
Sex will blow your mind.