Luo De Xiu on Xian Tien (pre- heaven AKA circular bagua zhang) training and application:

Xian Tian (pre-heaven) is the “integration system” of the Ba Gua Zhang method. It integrates all the skills that were trained in the Hou Tien (post heaven) (straight) lines. Synergizes the jin, jing, fighting skill etc, and trains it at a higher level, specifically working on training and integrating the Yi into the fighting skill.
"It’s like chemistry, you take two different elements and mix them together to create a third new element.”