Shifting weight in behind the movment can even be done many ways:
with the rear heel comming up slightly or both feet moving into postion at the time of the landing, (a slight body lean ! but its not recomended at all ;unless your going over the top outside of someones lead arm so you are safe for that second), or by a very slight hip flex as you shift sit on the punch. A bit of all (except the lean) is effective and explosive.
Vertical fist and horizontal fist have differing effects on all of the movments too, one creates a more upright alighnment of the head but only slightly maybe a 1/2 inch. Vertical fist or angled fist so your elbow is down : if in the opposite stance to your opponent.. his left jab countered by your right jab vertical over the top of it withthe above footwork also gets you out of the way of his and puts more power into yours as well as deflecting his.