While performing the sexual act of double penetration a male participant slips out of the female's vaginal or anal orifice. The other retains his position. While attempting reinsertion, the expelled male forcefully jabs his penis into the other male's scrotum. There by, "stabbing the bag." Commonly viewed in porn pornography.
"Father Harry won't swing with us anymore because my husband stabbed his bag."
"I enjoyed DPing Molly with you... Until you stabbed my bag."
"This must be amateur porn; that guy keeps stabbing the bag."