Sha (~) is the antithesis of Ch’i and can be translated as “noxious vapor.” It is a form of evil Ch’i and is often called sha ch’i, or feng sha (noxious wind).
Sha can be produced by a configuration of the landform that leads to the loss of good Ch’i or actively promotes evil ch’i, or it can be generated by conflicting influences and conjunctions as determined by the Compass. It can also literally refer to a cold wind issuing from the earth (marked by hollows) or through gaps in the protecting mountain ranges to disturb the accumulation of ch’i and render the site inauspicious.
“Secret arrows” are straight lines which by virtue of their power to conduct ch’i pierce any accumulations of ch’i and reduce its efficacy. These lines can be straight ridges, house tops, railway embankments, telegraph wires, or any set of parallel straight lines aimed at the spot that is subject to the evil influence of these “secret arrows.” These are a specific form of sha. Such “secret arrows” can be blocked off from a site by a wall, row of trees, embankment, or octagonal “target boards” with the appropriate deflecting characters written thereon.
Source: Feng Shui The Living Earth Manual page 45/46 Dr Stephen Skinner