Fengshui is basically the measurement and manipulation of the flow of qi, the Chinese over the millennia have developed efficacious methods for deflecting and rechanneling harmful qi (called sha).
Harmful qi is of various kinds. In general qi that represents a balance between yin and yang is auspicious, whereas imbalanced qi–that which contains either too much yang or too much yin, is inauspicious. For example, rapid qi is inauspicious (fast flowing water, high winds), and so is torpid qi (swamps, doldrums). Any natural or artificial feature which would cause the flow of qi to become imbalanced is inappropriate. For example, a straight road or path leading to the front door of a house would allow the ingress and egress of qi to be too rapid. A means of breaking up this rapid flow is necessary if bad fortune is to be avoided. Rapid qi, such as that created by the barrel-like “gun” of a straight road aimed at a front door, can be moderated by bells or chimes hung at the entrance, or reflected by mirrors flanking the door. On the other hand, it is inadvisable to build a house with the entrance directly facing a monolithic feature such as a large tree, a cliff face, or a taller building, because the flow of qi would be slowed too extensively. A means of opening up the obstructed qi is necessary, or misfortune for the residents of such a house is probable.