The role of placing a long knife on the northwest bedside in geomantic omen

The northwest belongs to the cathode, and one idea is that the main knife can deter the suffocation produced by the main northwest. But subjectively, although tools have deterrent effect, they are regarded as a kind of energy and negative energy. If this energy can belong to the owner, it can benefit the owner, but it can't belong to the owner, but it is extremely harmful.