The negative electrode provides protection to what ever you may be trying to have an effect on. This protection is stronger close to the negative electrode but it extends to 1/2 the distance between the positive and the negative electrode. If you place the negative on the back and the positive on the front, this protection extends half way through the chest.
If you place the negative electrode in the left hand and the positive slightly below and to the right of the heart, then all of the protection area will be in the arm and you will get better coverage.