Either when your balls smack against your girlfriend and bruise from being too rough, or when your girlfriend bites down too hard when giving you head and you give her her a punch in the gut to correct her mistake.
2. Sex that is too rough for either party and results in possible injury. Comparable to paintball without clothes on.
Steve- I was about to give her a dirty sanchez but it turned into paintball sex after she bit down.
John- Too right buddy I practically puked after my balls hit Megan too hard last night.