The guy seats himself on the motorcycle as usual, and the woman mounts him by sitting on his lap so they are face to face. She will need to wrap her arms around him and hold tight to maintain balance. If she has rear foot pegs to brace against, the woman can control the depth and speed of penetration by riding the man's cock and bouncing up and down on it. You don't want to bounce too much, though, or you'll wear out the shocks - or fall off the bike. This is the best position for the guy if he wants to actually see where he's going.