When a column of water is moving and the flow is shut off abruptly then the force of motion of the water continues in the form of a kinetic wave. This wave travels quickly, faster than the speed of sound. The result of this is that pipes can make loud noises and if the pressure of the wave exceeds their capacity then they burst, sometimes with fatal effect.
The human body is mostly water.
If you surge forward and stop suddenly you can observe the kinetic wave continue.
This results in a wave of pressure and force. The momentum of the body can be transferred into this wave, which will not travel through tense muscle, so the body must be relaxed and fluid.
One of the key parts to this type of force is the shutting off of the valve. The time it takes to shut off the valve is a major factor in the amount of force that the wave contains. This type of force cannot be a function of muscular strength, which utilizes totally different methods of actuation in terms of kinetic impact.
Internal martial arts utilize this water hammer principal. This occurs by making the body unified as a whole, the rules of the harmonies relates to this, the body arrives together, the upper matching the lower, the left and right sides reciprocal. The rooting provides a brace that ensures the force is propagated and not dissipated. The pressure of movement is converted into a force totally different than movement itself.
To slap a heavy bag with the water hammer type energy does not result in the bag swinging like it has been punched, but the bag, and everything supporting it will shake with impact. The noise of impact is also much louder than with blunt impact type strikes.
If you hit a street light pole with this energy the pole will vibrate and move and your hand will not feel more than a slap sensation. Do it wrong and your hand will feel like it just hit non-moving steel and you may become injured, The key is the change of momentum into a wave of force that propagates at a much higher frequency than the initial momentum. This requires relaxation and proper technique, one does not merely hit things with a heavy hand, rather the method is subtle and involves shutting off the gate of momentum, while relaxed, thus allowing the wave of force to continue.
To the untrained eye the use of this force will not be detected.
On heavy bags the blow will seem weaker, because the force is propagated at such a high speed the bag quivers but does not swing as it does when hit or kicked using external energy. However the force does not move the bag so much as travel through the bag. In terms of martial use consider the skull, to impact the skull with a heavy external blow moves the entire skull, the same with the body. The power of impact becomes transferred into the momentum of the object. However if one uses the water hammer type energy the energy of impact does not move the skull, but goes into it. This can result in greater damage than rote impact physics, in ballistic terms the shockwaves are called hydrodynamic shock and this is an aspect of the water hammer effect.
If you train in water itself you can test the principal by slapping water with various methods, instead of displacing the water you want to send a pulse or wave through it. This is the same as with people and if you can direct the force then you can literally target internal organs. Instead of wasting time compressing the ribs, you can send energy right through them. While an external blow leaves a bruise on impact, because force is being absorbed that way, the internal blow leaves little if any mark, often just a fading red mark, because instead of the force being dissipated by impact it continues through.
The physical dynamics of this method are counter intuitive, rather than driving through the target, as is often the case with impact methods, one stops at contact, letting the force itself continue, not the weapon or limb involved. If the impacting body is rigid then it will absorb the force of the wave and the wave will be poorly propagated. Instead it must be as water, only the fluid softness of water will allow such energy to be propagated and transmitted.