《Zi Bai Jue 紫白訣》is a commonly considered as one of the must-read classical literature for San Yuan 三元 feng shui practitioners and followers. Though saying so, San Ho 三合 派 feng shui stream has long histoty for incorporating the《Zi Bai Jue 紫白訣》into their methodology, that was the reason this literature being included into the San Ho classical fengshui book named 《Yang Zhai Ji Cheng 陽宅集成》written in Ching.Dynasty.
The essence of the 《Zi Bai Jue 紫白訣》can be seen from the first several sentences at the beginning:
紫白飛宮，辨生旺退煞之用。
三元分運，判盛衰興廢之時。
生旺宜與，運未來而仍替。
退煞當廢，運方交而尚榮。
總以氣運為之君，而吉凶隨之變化。
The above sentences could be translated into English as follows:
Zi-Bai flies over the palaces (9 palaces), would enable the differentiation of 生 (Create, Light-up) 旺 (Prosperous) 退 (Retreat, Drawback) 煞 (Destroy, Killing) energies into use.
紫白飛宮，辨生旺退煞之用。
San Yuan 三元 are further split into Yun 運，so as to judge the timing period of 盛 (Boom) ,  衰 (weak), 興 (Rise-up), 廢 (Abandon, Obsolete)
三元分運，判盛衰興廢之時。 
The Zi-Bai 生旺directions are better at the timely Yuan. If not at timely Yuan would not trigger the advantages.
生旺宜與，運未來而仍替。
The Zi-Bai 退煞 directions should be abandoned, however if they are at the timely Yuan, would be still of the benefits.
退煞當廢，運方交而尚榮。
In conclusion, Yuan is the master key to drive the good or bad，luck or disaster accordingly.
總以氣運為之君，而吉凶隨之變化。