Venous leak
Encyclopedia
An erection
Erection
Penile erection is a physiological phenomenon where the penis becomes enlarged and firm. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal...

 is produced when blood to the penis compress the veins in the corpora cavernosa so the blood cannot leave.
Venous leakage is a result of these veins not closing well enough to retain the blood, and the erection is lost.

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