At the point of contact, you want to be able to generate at least two energies: one attacking the structure and one attacking the mass. The why and how of this may not be immediately obvious, but the principle behind this “fork” energy can be understood via simple models.
As a first pass to understanding, we remove complicating factors and reduce our analysis to the essentials needed for examining the underlying principles. Using a “spherical cow” approximation technique, we can do barebones thought experiments to derive the mechanisms of the fork energy.
First, we start with some basic definitions. The structure would be the solid frame of the body, i.e. the bones of the skeleton. The mass is the effective center of mass of the body. For simplicity, we’ll ignore the multiple “centers” which can be attacked and just focus on the overall center of mass. The center of mass is the mathematical average position of the body’s mass.
This center of mass is a convenient point for physics calculations. The effect of any uniform force field (like gravity) can be computed with respect to this center of mass point instead of going through the hassle of integrating the force over the entire volume. We do not generate a uniform force field in martial arts outside of video game or movie contexts. However, we can still consider force applied to the center of mass as affecting the whole body.
Applying a force directly to the center of mass is not generally a feasible option. No one except friends who are touchy-feely comfortable with you are going to let you apply force to their center of mass in a direct fashion. Most people are going to defend themselves, and that means your point of contact for applying force will often be the arms. Even when applied in defense, the extremities are attached to the body; applying force to the arms can still affect the body. Take for instance the rigid structure example. With a rigid structure, the arms are directly and statically coupled to the mass. Consequently, force at the point of contact at the arms can be aimed directly to the center of mass. Applying force toward the center of mass from the distant arm point of contact has the same effect as applying force more directly adjacent to the center.
Applying a force to the center of mass via rigid arms is the easy example, but what happens when we don’t have such simple conditions. Let’s say we have to deal with noodly arms.
Without a static rigid structure, engaging the mass becomes a more complex problem. Dealing with His Noodliness might be impossible for mere mortals. However, when dealing with regular humans, it is still possible to apply a force to the center of mass with contact on non-rigid arms. Even with loose arms, there are still solid structures which can be linked together. We can model the arms as two solid rods serially connected to the body via a hinge joint (elbow) and ball joint (shoulder). Attacking the mass through the arms becomes a problem of linking the arm structure to the body through the joints.
With the lack of tension to hold the arm in place, directly attacking the center of mass from a point of contact on the arms causes motion at two joints. The arm motion decouples the arms from the mass making it nearly impossible to apply force to the center of mass. To affect the mass from the arms, force must be applied through the structure first to link the bones through the joints. Only after the bones are linked together can a force be applied to the center of mass via the point of contact.
Manifesting a fork energy requires generating multiple force vectors from the point of contact. Those force vectors have to be aimed in the correct direction to knock the structure into a stable configuration and attack the mass. Generating the proper force vectors requires a fair amount of body control and enough sensitivity to feel the positions of the structure and mass. Assuming the force vectors can be generated and pointed in the correct directions, the forces then need to be applied with the correct magnitude and timing. You cannot just blast a force through the structure to link it together. Each joint will have a certain amount of resistance holding it in place. This resistance is the sum of the applied force, friction in the joint, muscular tension, and connections from ligaments, tendons, and fascia. Too little force will not knock the bone into the joint enough to stabilize the structure at the joint. On the other hand, too much force will overwhelm the other components holding the joint position and bump the bones and joints away from their linked position.
Once bones have been linked at a joint, the force direction must be changed with the correct timing to knock the next bone and joint in the chain. Once a force has been applied to link bones at a joint, there is a window of time in which the stabilized joint can be maintained. Knocking the bones together at a joint is only a temporary stable link which depends on how much resistance there is at the joint. If we subtract out muscular tension, the remaining major contributors to holding the joint position are friction at the joint and the soft tissue in and around the joint. Both of those will exhibit a time-varying response under a force load and will drift away from the induced stabilized joint position. The less tension in the system, the smaller the window of time in which the joint will remain in its stabilized position. The next structural segment has to knocked before the bones drift away from their stable position.
If the structure attack is successful and the bones are knocked into a stable configuration, the linking of the bones and joints needs to be maintained while a force is initiated to attack the center of mass. If the structural lock disappears while attacking the mass, we are back to square one with contact on an arm which has been decoupled from the center of mass. When both the engagements of the structure and mass are manifested, the energy at the point of contact “forks” along two different paths.
Disclaimers:
- Figures not drawn to scale
- Semblances to pop culture characters purely a happy co-incidence of bad artwork skills.