Kickback is created when the rotation of the hips essentially pulls the heel from the ground and onto the toe. In some hitters (See Arenaudo's rear foot) this is much more visible in that you can see the rear heel pull behind the lead foot and in other hitters this move is much less pronounced as there is less of a heel "slide". This move is the byproduct of a strong hip to heel connection and firing sequence of the rear hip/adductors that accelerates the torso. Hitters that maximize the force that they put through the ground do so in a way that connects through their feet.
Looking at the rear foot, you can see that the hitters in each of the videos highlighted here load against their rear leg in a way that puts them into the center of their foot. This is the most powerful and balanced position we can work through as we begin our loading sequence. As the hips begin to load, the tension begins to spiral down through the leg, loading the glutes and adductors putting the force through the rear hip. This tension is what elite hitters are looking to use as the "engine" for force production in the swing.
Maximizing Ground Reaction and the Pull Back
One of the more interesting comparisons to the kickback in a baseball swing is that in a hockey slap shot. What we see occurring in the images above represent an interesting dynamic between the lead leg and the rear leg during the initial firing phase of the sequence. Some have referred to this movement as a "scissor" effect in that you can see the lead leg "pull back" as evidence of the lead leg working from slightly flexed to more extension from the knee. This pull back from the lead foot and leg allows for the acceleration of the rear hip. A good analogy for this dynamic is the braking system on a bicycle. If you are riding down a hill and hit the front breaks on the bike, the rear tire launches forward sending the bike and rider end over end. When the lead foot makes contact with the ground, it essentially pulls against the ground causing the lead hip to pull back and the rear hip to accelerate forward. The more efficient the lead leg is in "bracing" or braking, the more efficient the transfer of force through the lead leg.
One other key feature that we see with strong kickback is the direction of the hips. While it is imperative that the hips fully clear, hitters do not want to have excessive pull with the lead hip. As part of the separation between the hips and shoulders, the lead hip begins to open as a function of the rear hip begins to turn and the rear leg internally rotates. If the lead hip begins to pull across or away from the strike zone, there will be an eventual pull on the front shoulder that can limit coverage on the outer third of the plate. Taking this action a step further, we can look at the hockey slap shot and compare it to the hip extension and top hand "release" int he baseball swing. Because the skaters toes are lined up forward, in order to keep the shot on target the shooter must work across their body, any excessive pull from the front hip would cause the shooter to make an off-balanced shot.
When we look at the image of Trout above, we can see that the pinching action of the rear and lead leg allows the lead hip to remain closed longer during the stride. This in turn allows the force to be produced in a more linear direction before you see the hips fully clear. Again, trout is one of the more prolific hitters that displays this action but if you look at the patterns of elite hitters, the force generated through the feet pulls upward from the ground causing the heel kickback.