Venous leak occurs when the veins in the penis fail to properly trap blood during an erection. Under normal circumstances, when a man becomes sexually aroused, blood flows into the penile chambers (corpora cavernosa), causing the penis to expand and stiffen. At the same time, veins that usually drain blood from the penis constrict to keep the blood trapped inside, maintaining the erection.
Source: https://www.dotclinics.pk/venous-leakage