The common knowledge 8 Mansion Feng Shui (or for the matter is called 8 Mansion) is because of the 8 direction in a BaGua or compass.
The common application of 8 Mansion Feng Shui （八宅風水）is using there person year of birth to derive the Gua Number, thereafter plot out the 8 direction. Other approaches includes using the main door as Stability direction （伏位）or the use of the facing/ sitting direction as Stability direction （伏位）to plot the rest of the 7 direction.
With that, most online calculator will show you the 4 auspicious and 4 inauspicious direction. What we do with this information is that, we manage the risk by placing ourselves to tap into the energy of the auspicious direction; and reduce the exposures to the inauspicious direction – Risk Management in a broad sense, thus ‘seeing the forest and not the trees’.
The Chinese LuoPan has subdivide the each of the 8 direction with 3 subsection. This makes up the 24 mountain of the LuoPan. The classic text spoke about NaJia （納甲）, which relates to the change in lines of the hexagram (Gua). Applying the concept of NaJia on 8 Mansion Feng Shui, the practitioner will arrive at the 24 Mountain Qi Chart for 8 Mansion Feng Shui. Allowing the practitioner to see the forest as well as the trees.
坎纳癸，( 申，子，辰 )
离纳壬，( 寅，午，戌 )
兑纳丁，( 巳，酉，丑 )
In 8 Mansion Feng Shui, the various sectors has different level of energy. I’ll use a simple scoring system to ” + / – ” to illustrate the point.
For example, a Gua 3 （震） person or house. The 生氣，Life Generating is at South. This does not mean straight away South sector scores +80. Instead, using the Chinese LuoPan, South sector has 3 Mountains, namely Bing, Wu, Ding （丙，午，丁）.
Bing （丙） comes under Gen （艮）, –70，六煞，Six Killings
Wu （午） comes under Li （離）, +80，生氣，Life Generating.
Ding （丁）comes under Dui （兌）, –90，絕命，Life Threatening.
And, so it seems… Not all the 3 Mountains in South direction is auspicious. The rest of the sectors, readers may easily extrapolate using my example (above). In fact, if the scoring is aggregated, it is a net deficit. This explains why Life Generating sector or direction fail to work as claim by Practitioner and Feng Shui Masters. This is just the NaJia （納甲）aspect of 8 Mansion （八宅）. I haven’t touch on Qi （氣口）, Water Methods （水法）and TaiSui （太歲）. Hope this help to illustrate the point and level up your application of 8 Mansion Feng Shui （八宅風水）.