I just watched a show / infomercial on HECS. I had never head of it until the other night. The reasoning sounds believable, BUT...There are animals, like sharks, rays, and fish that have special electro-sensitive organs. In these cases, the animals are sensitive to bio-generated electrical fields out to distances to 3' or less, and more like 18". And water, especially salt water, is a vastly better conductor of electricity than air.
Is it hype and snake oil? Maybe not, but if it works, the mechanism may not be from foiling animals' electro-sensitivity. From what I saw, I can't rule out that it was the Mossy Oak or Real Tree pattern that fooled the animals. A double blind test, where results from hunters, over several seasons, who don't know if they're wearing HECS technology or not, would tell the tale, and seal the deal.