Ball brushing can occur at many stages during an act such as a threesome, where both the males are exploiting the lower region/s of the female. In the somewhat cramped conditions of the vagina during a threesome, it is easy to look control of the oscillation of you testicles. Careless fluctuation of the 'bollocks' can lead to a sometimes unnavoidable case of ball brushing.
Male 1: Yer! say my name bitch!
Female: Oh fuck yer, give it to me Male 1
Male 2: Urgh!! Shit man mind the ball brushing!
Male 1: Fuck! Sorry dude my bad, I got carried away