It means the activity of putting one's face between a woman's breasts, and rocking turning one's head rapidly from side to side while making a noise like a motorboat.
"Put your head between her tits and go bbl bbl bbl bbl bbl..."
The placement of one's face, specifically the mouth, into the area between a well-endowed woman's breasts, followed by a rapid shaking of the face in a side-to-side motion accompanied by yelling. The resulting sound that is created sounds similar to an outboard boat motor.
Placing your mouth in between the butt cheeks and viciously and aggressively shaking your head back and forth while making the sounds of an out board motor (comparable to motorboating tits).