The CC and CS are the internal chambers of the penis, they fill with blood during an erection. Read this:
glans - the “head” of the penis, normally covered by the prepuce (foreskin) it is abnormally exposed by circumcision
body - the “shaft” of the penis mainly containing two corpora cavernosa and the corpus spongiosum
urethra - the tube through which urine and semen exit the body, contained within the corpus spongiosum
meatus - [me-ate’-us] the opening of the urethra at the end (usually) of the penis, an abnormally placed meatus is called “hypospadias.”
scrotum - the “ball sack” or “nut sack,” a pouch of skin that contains the testicles. It is separated into two chambers by a septum so your testes and supporting structures don’t get twisted around each other.
corpus spongiosum - a sheath of spongy tissue which fills with blood during an erection and surrounds the urethra, usually abbreviated “CS.” It expands at both ends. At the base the expansion is called the “bulb” of the penis. At the other end it is called the glans. Blood within the CS can flow freely between the bulb and the glans.
corpus cavernosum - one of two (plural is “corpora cavernosa”) tubes of spongy tissue that fill with blood during an erection, usually abbreviated “CC.”
tunica albuginea - a tough sheath of connective tissue in two layers, one surrounds each corpus cavernosum and forms the septum between the two, the other layer surrounds the the CC and the CS. The spongy erectile tissue in the CC would not cause an erection without the tunica because the skin covering the shaft is able to expand. Because of its tough fibrous nature, the tunica resists further expansion after the CC are filled with blood and, under the pressure of the contained blood, forms the rigid structure that is an erection.
deep fascia (Buck’s fascia) - the deep fascia of the penis is a connective tissue sheath that surrounds the CC (and tunica), the CS, and the major nerves, arteries and veins of the penis. The superficial dorsal vein of the penis is outside this sheath. The deep fascia extends from the corona of the glans to the fascia covering the pubic bone and is covered by the skin.
fundiform ligament - this ligament supports the penis in a sling-like fashion. It originates in the lower abdominal wall and splits to surround the base of the penis where it attaches to the deep fascia.
suspensory ligament - this strong band of tissue originates at the pubic bone and splits to connect to the deep fascia of the penis. It does not attach to the tunica.
Ok, do this for me, get a glass and fill it with water, then add more water to it. Its going to overflow as it was full to begin with. This is the best analogy I can give as to why you only jelq with a 40 to 80% erection level, you have room to bring in more blood and stretch those chambers.
I hope this was helpful and not clear as mud.