Officially it's called testicular torsion. Basically your testicle gets turned so the veins to it get twisted and the blood flow is hindered. It can lead to permanent damage and removal of damaged testicle if left untreated. I've had it once in my life (not at the moment, sorry for baites :( ), and it's extremely painful, top3 pains in my life. Didn't get permanent injuries thankfully.