Following are reference articles and research should you decide to further investigate and learn about PEMF for your horses
Dr. Oz and his explanation of PEMF
An article published by the Draft Horse Journal.

You can research, on the internet lots and lots of PEMF articles.  Here are a few to get you started and to perhaps answer several of your questions.
From the Holistic Horse
Search horse PEMF on You Tube and you will find lots of information
US National Library of Medicine
An article by Dr. Robert Jay Rowan