Interferential Electro-Therapy uses electrical pulses to stimulate nerves and muscles in the body, to help relieve pain. Electro Therapy was first developed with the introduction of TENS machines. TENS means Transcutaneous Electronic Nerve Stimulation. Basically an electrical pulse is transmitted through the skin layers using a frequency modulated sine wave. However TENS uses a frequency modulation that causes some resistance to penetrate the skin layer, this can result in an unpleasant feeling to achieve any possible benefit.
Interferential machines have a much stronger carrier sine wave that makes penetration of the skin easier, the sensation is more pleasant and comfortable than TENS. Also because the signal is stronger it penetrates much deeper than TENS, giving a better and longer lasting effect.
The reason Interferential works so well is that by crossing the electro-leads the two currents produced by each channel actually interfere with each other to create a pulse beat that penetrates deep into the muscle and tissue. Tens technology does not have this capability. It could be reasonably argued that Interferential goes beyond TENS technology.