Q: So imagine I have a TENS pad on my belly, and another Tens pad on my back. Is the current:
a: Passing through my intestines to form a circuit?
b: Following the path of least resistance and going around the skin?
If it's b then how can contact mode be useful for treating conditions deep inside the body?
A: Obviously, the current flows through the least resistance path. This is a fact. However, the exact path it is actually using is unknown.
The human skin is a good isolator, so it is easier for the current to go through the interior tissues than travel through your skin, once the skin barrier has been overcome.
However, there are a great number of factor to be considered, as the layers of body fat (very isolating, much more than skin), the actual body percentage of water, the electrolyte levels, even the stress suffered by the person itself.
Generally speaking, the belly area is prone to have a fat layer beneath, so you can find a place to attach the tens pad that follows the rule of be placed across the organ, but at the same time, find a place with little fat, to allow current penetration.
Also the current does not penetrate on hollow organs (according to Hulda Clark) nor in the inside of the eye (because of the very isolating membranes surrounding it). So, if you need the current penetrate into a hollow organ, lets say your small intestine, it is a good idea to fill it with some conductive substance before the treatment. Water is very conductive, and must be taken abundantly to enhance overall conductivity, as well as to fill the digestive tract (if you need to treat it).
A German practitioner, Alan Baklayan, has investigated profusely the best electrode placement for each disease, and he mostly uses arms and legs, because if you place the electrodes too close, the difference between the electrical resistance of the skin and the inside tissues is low, but if you place the electrodes far away, the difference increases a lot, and the chances for the current to travel through your outside layer of the skin diminishes to almost zero.
I have experienced a great conductivity increase by using moisturizing cream in the exact zone where the tens pad are going to be placed (and wait until the cream is totally absorbed). This is a good practice if you usually use your ankles, or a dry portion of your skin. The increase of conductivity can easily be measured with a multimeter.