Appears to be more of a training device rather than something an experienced shooter would want. My shoulder hold placement changes with each position I am in and what ever body armor I might be wearing will also change positions. The magnets might work great if shooting from a bench rest position once you get the body armor adjusted right.
This concept might be much better if one magnet was drilled in to the cheek stock and the other drilled into your cheek. Getting your head against the stock in the right place is far more important than where the butt plate is.