Ok Dan.....this is pretty rudementary, but you'll get the idea, I think.

Picture 1 is a level, sitting perfectly level to the ground. The blue circle represents the bubble. Now, roll that cube forward, on the pink line. If it doesn't roll perfectly level, (3rd axis) the bubble will look like the 2nd picture. You will then correct the level, causing the bow to be canted badly, and miss right/left. Hope this works, my paint skills suck.