Tying off keeps the blood in not out---keeping unit engorged---for aesthetic purposes...
It makes no difference whether you keep the blood in versus out Park..
Your penis contains highly vascular tissue all around (Corpus Cavernosum) which needs to have a constant amount of fresh, oxygenated blood in order to not have the tissue die of hypoxia. While flaccid, it does remain in a hypo-oxygenated state, but once an erection occurs, there is a sensitive exchange of oxygen-rich blood that must take its course.
Once you trap the arterial blood into the penis shaft, the actual oxygen within that sample of blood is used by the surrounding tissue. By way of diffusion, hemoglobin picks up the oxygen from the arterial blood, in turn the surrounding tissue then release carbon dioxide (a by-product of diffusion) that needs to be returned to the heart via venous blood flow. What remains after that exchange of oxygen and carbon dioxide is blood that is not fresh in oxygen which can not adequately nourish the surrounding tissue.
Essentially, by "tying-it-off" you are pretty much starving the penile tissue from the oxygen that it needs to stay alive.
That man's penis was morphing into a purple crayon,
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1"
P.S. For the Jews and circumscribed crowd, think of what happens when your foreskin is tied in the Plastibell for a long enough period. The tissue just dies from lack of circulation and will even fall off...
