It is refered to doing graffiti on the outside of moving bus or train.
Dude! Check him out!!! He's motion tagging the bus!! HOLY SHIT HIS SHIRT IS CAUGHT ON THE DOOR!! OOOH DAMN!! THE BUSS IS DRAGGING HIM DOWN THE STREET! THE BUS IS MAKING A LEFT TURN AND HE IS BEING PULLED UNDER THE BUS!! MY GOD!! HE'S TAGGING UNDER THE BUS!! GODDAMN! THATS SOME HARDCORE RAMBO SHIT!!
by Freash off the Boondocks February 27, 2004