It is done by magnetic induction lines under homes. If you go look at some of your power poles. There is a grounding wire running down should be unshielded. At ground level if you see a shielded wire running away from the pole into the ground. They have a box or pipe design and use it to cause a magnetic field effect. Ive also found my walls are highly affected at midpoints by using a simple app to detect gauss in tesla meter.