The assault of one's testicles from behind. Generally, the victim is bent over, however a standing attack up the legs is also commonplace.
Often times, the assailant will yell "back sack attack!" either at contact, or after the strike is complete.
Winding up for a kick, Ben positioned himself behind a bent-over Matt. Using mostly the flick of his foot unto the scrotum, Ben yelled "Back sack attack!" before Matt ever got the chance to react. Devolving into his fetal position, Matt sat writhing in pain.