Out of curiosity, I looked into this a little. The best description I found in my limited research came from a company that sells all kinds of TENS and muscle stimulation units. I think this Marc Pro is basically an Electronic Muscle Stimulator (EMS). There is a difference between the TENS and EMS:
How to Choose Between an Electronic Muscle Stimulator and TENS Unit
Electronic Muscle Stimulators technically are TENS (Transcutaneous Electrical Nerve Stimulators) devices, the difference being which nerves in your body are stimulated during electrotherapy. EMS Machines are designed to stimulate muscle motor nerves while TENS devices stimulate sensory nerve endings. Both types of treatment work by generating electrical impulses that stimulate nerves through the skin causing muscles controlled by those nerves to react by contracting.
With that being said, there are WAY cheaper ways to go than spending $600 on an EMS. Here is a link to a combination TENS/EMS unit for $65 on this site:
I don't think you could go wrong with this.