Essentially, clamping serves to initiate a slow progression into localized hypoxia because it allows more blood into the penis while allowing less blood out of the penis. This is why such intensity or pressure differential is possible with that technique. No matter how much blood you force into the clamped penis you are still far below the normal circulation or exchange rate that takes place without the constriction or clamp in place. While you are forcing more and more blood into the clamped penis, you are certainly not anywhere near supplying the same amount of "new" blood that arrives in the normal state. This is what leads to the localized hypoxia...and then the resultant body's defensive reaction.
The defensive reaction of vasodilation is an attempt to increase blood flow into the penis. This is primarily accomplished by a sudden release of histamine by "mast cells" in the tissue under distress. (as well as a release of, or production of, nitric oxide to relax "smooth muscle" tissue...slower process than the histamine release) Anyway, this process is well known. It is most often associated with applying pressure to tissue or even to arteries to cause localized hypoxia with the intent of artificially and "mechanically" inducing vasodilation. "Ischemic compression" is widely used to both cause vasodilation and to cause a large burst of "stored" or "waiting" blood and nutrients to "flush" the area once the force is removed.
Now so that all of this blather is related to this particular thread somehow while still being able to tie in with the above. (But could add to the above with clampers working in what follows)

As to jelqing and the related and similar reactions to that type of "engorgement" exercise. The forceful "mechanical" movement of the blood into the penis also causes vasodilation. However, the histamine production is even higher for this forced flow versus simply static engorgement, over 120% increase in histamine is achieved during exercise. (It is a very rapid release of histamine instigated by nerves sending impulses to "mast cells" to release this chemical) Furthermore, this elevated histamine level remains for many hours after exercise is stopped.
Furthermore, if one is to stretch at the end of the jelq stroke, which is advisable, this force also adds to the goodies.

Stretching the tissues has a vasoconstrictive effect. What! That is bad! LOL Yes, for our purposes it is. BUT the vasoconstrictive effect only last as long as the force is applied in the [words=https://shop.mattersofsize.com/products/sizegenetics-penis-extender]traction[/words] phase, or stretch. As soon as the force is released, vasodilation again rears its head in response in order to "protect" the cells. The most important part of this "stretching" effect is that the nerves in that area "learn" not to overreact with repetition. What happens, with dedicated and routine practice like all of us Penis Enlargementers practice , is that every stretch's vasoconstriction effect lasts shorter and shorter and shorter the more normal the stretching becomes. And the icing on the cake? The vasodilation that occurs when the force is removed still happens no matter how short the vasoconstriction duration becomes.