In Wood (Beng), the front hand downward blocks much like Tai Chi Brush Knee, which pulls the rear striking hand which shoots out like an arrow.
The front hand’s brush knee type downward block and forward step are propelled with the forward drop; which is controlled by the rear foot and focused with the front foot.
The brush knee and forward foot movement propel the rear reverse punch arrow hand.
More on Xingyi
From: Axe Hand; Hsing-i & Internal Strength Workout