In some young men the tissues that surround the testicle (testis) in the scrotum are loose. Therefore, the testes can move around in the scrotum more than usual. This can be called a 'bell-clapper' deformity. If a testis twists around, the spermatic cord has to twist around too as it is fixed higher up. If this happens, the blood flow to the testicle is blocked in the twisted spermatic cord.
This is a companion discussion topic for the original article at https://patient.info/mens-health/scrotal-lumps-pain-and-swelling/torsion-of-the-testis