Many factors not limited to better circulation, a penis that is more conditioned to being exercised in that manner (such as mine) and a strong PC muscle to perform kegels and push that new blood in. Other aspects are technique mainly along with the arousal level which must be maintained.
Clamping, Constricting, Uli's, Tourniquet or whatever else its branded as will work and bring real gains but it has to be mastered and isnt a case of slapping a band on and sitting with an erection like some men think. You have to keep aroused which can be difficult at times, peform the exercises you want for the purpose your Penis Enlargement'ing for i.e. head gains, length or girth etc.
What I have found is in time with experience of expanding the penis with intense girth methods and just general penis enlargement overall is that the penis becomes not just larger but its capacity to stay thicker or larger longer is increased ... I see this to like endurance enhancement to the penis like a muscle is enhanced with its endurance to last longer at X amount of force, it isn't just about the size and strength.
So with me now I notice my erection and especially recently the semi erection looks fuller and lasts longer. The 'endurance' if you like to the penis increases and in time you see greater size with sessions which in tail lead to more gains and permanent results.