Interferential uses two channels set at different frequencies. The usual setup is shown below.
Interferential Setup – The object is to surround the problem area, usually in an “X” pattern. Such as side to side and front to back or side to side and top to bottom. The point is this way you can treat a large area such as several vertebrae segments. I recently used it to treat spasmy muscles in the perineal area which was causing pain. So I piked two frequencies which both reduced pain and caused muscles to relax. And it worked when the doctors could neither diagnose the problem nor treat it. So much for doctors.
Now with Frequency Specific Microcurrent (FSM) they use Interferential somewhat differently. They connect the + leads from each channel together on one electrode and the – leads onto the other electrode. They then place the electrodes in such a way as to make the current go through the problem area. In both setups, the two channels interact similar to waves interact in the ocean. The result is that the tissues see the frequency sweep between F1 and F2 at the frequency of F1-F2 which is called the beat frequency. And the whole area sees this so no adaptation occurs. The main effects in a normal interferential treatment is seen at F1 and F2 with a slight effect seen at the beat frequency. In FSM the effects are more complicated than that. I use FSM quite a lot in my practice and use normal interferential when appropriate.