Think about a joystick being on the inverted setting, (up is down, down is up), basically what ever your muscles "remember" is what they will do.
So, with a mouse, if to move to the right, instead of moving exactly along the x-axis at 0* in the positive direction, you move 20* above x-axis, but it still goes in a straight line.
Notice how if you dont move your whole arm, but instead use the lower palm of your hand as the pivot point, and move from left to right, like in a curve/half circle, that the pointer still goes across the screen straight. The angle of the mouse changes constantly, but the line stays straight because the pad underneath is moving perpendicular when it is compared to the reference frame of the mouse.
Originally Posted by Cavi Mike
You could hold the thing facing backwards and it won't matter. All that matters is when you need to put the pointer somewhere on the screen, you can put it there without having to think about it. Do you think people with feet pointing outwards can't walk in a straight line?