Xingyi steps first.
Xingyi is the only method for adding the force of the step to your strike with the Scissor Stepping: which not only focuses the whole body weight, but lets you 'hit with the ground'.
When you are driving a car, if you put on the brakes slightly before stop, the car will jerk forward a little, before it stops and hit with a lot of force.
If you are moving forward, and halt your step just before you hit something; it will add to the destruction since it propels your whole body weight forward.
When playing basketball, and someone stops a little too late before colliding, it will send the other player flying.
The Xing Yi Scissor Stepping is rooted as a sail boat's keel is. When a boat rocks, it will send a wave of motion upward to the mast; whipping the tip.
A common internal concept is keeping feeling in the rear foot. This feeling, should be based toward the heel, and relates to rooting; and the propagation of the wave energy from the ground, throught the legs, directed by the Lower Dan Tien via the scissor-stepping unique to Hsing-i (Xingyi, Hsing-Yi).
In every movement, watch your Yi. When Yi generates the idea for movement, the Qi will be immediately led to the end section, starting the movement of the end section. The middle section follows and the root section urges the movement. This is not the same as Tai Chi, because the body in Hsing-i is more like rattan than water. Even though it is flexible, the body is hard so when the Yi is generated on the target, the tip can move first, and the power is pushed from the body and the root section.