An attempt during vaginal intercourse for one bro to secretly switch with another between thrusts. Thus the women has little to no idea that she is being railed by a different man.
"Catherine said you pounded her for over an hour last night! How did you do it?"
"Oh, my bro and I pulled a swap-in."
"Craig was doing this chick froggy style but starting passing out. Jon, unselfishly, swapped-in for him."