It would be nice for the manufacturer to identify how it works. Anyone please correct me if I am wrong, but here are a couple of points I have heard noted regarding this product. It is rated 15A, which is probably fine for many applications but most commercial circuits are 20A. I don't believe the Hum-X is UL listed, at least it is not so noted on the product data, which can be a significant concern for some applications. And according to others, the Hum-X uses power diodes such that in normal use there is effectively no connection to ground but if the potential gets high enough then the path is made, thus you theoretically have safety ground path only during a fault. The concern with this is whether the voltage that would trigger the safety ground path or that might pass through the device during a fault might also blow the diodes, thus breaking the safety ground path.
I looked at the Hum-X today and it is NOT UL listed.