Shan is similar to Brush Knee Twist Step. In the image beside the description you used Rubén, Yang Cheng Fu (A) is using his left palm to strike at B's neck/face and his right to draw aside (pie kai) B's left arm, dissolving the shoulder strike. This is a use of Lieh Split energy just like in Brush Knee, the waist turns right, the right arm drops and draws open, the left arm pushes forward.
It's hard to find good descriptions of Dalu. The internet is rife with videos of bad dalu performance. I think you've got the best book for it though, that and Fu Zhongwen's Mastering Yang Style Taijiquan also translated by Louis Swaim.