cryptorchism

An abnormal physical condition, involving the male reproductive glands, or the 'testes' if you will. It tends to manifest itself during the first stages of sexual arousal. The erectile tissue of the penis becomes filled with blood, engorging the penis to a semi-flaccid state. The testes at this point are withdrawn into the body by the arteria testicularis, frustrating the blood absorption process of the penis. The penis violently contracts as a result, contusing itself severely at the base. The spasms will increase until the entire stem of the penis has become purple or black-and-blue in color. Despite the excruciating pain, seminal discharge will begin to leak out of the glans (penis tip).

It's recommended to pull back the testes immediately, despite the debilitating pain, to prevent further complications to the male reproductive organ, such as testicular torsion. (Where the blood supply to the testicle is cut off because of 'twists' in the spermatic cord.)