HH with your high wing trainer as you know you can simply place your fingers under the wing and arrive at a point where the airplane will hang level and this is how we check where the CG is.

Now though what will happen when we turn that high wing airplane upside down and try to find that point? we will not be able to find that point simply because the airplane is top heavy. In other words it just will not balance and is too unstable to find the point.

When we have a low wing airplane we can find that point (in most cases) balancing it on our fingertips with it upside down, But agine now if we try to find that point with the low wing right side up it will be top heavy and too unstable to find that point.

John