The island is floating and has propellers. The ropes may not be used to actually pull, but to steer. There are two ropes so if you were to turn a corner, you would pull more on the inside rope which could then in turn make the inside propeller slow down and/or outside propeller speed up, therein turning the island. As she walks forward and the ropes pull tighter, this could act like a throttle cable, causing the propellers to speed up and when she stops, the ropes would go slack and cause the propellers to stop. So think of the ropes more as controls then leverage and it all becomes that little bit more plausible.