I will try to explain, as clearly as I can why I think there is so much confusion in this area. I will, just for background give you some of my credentials at the end.
In the scientific realm, and in any pursuit that attempts to prove or disprove a theory, there has to be 3 major elements present. 1. Common understanding of terms. 2. A large enough data set from which to analyze results. 3. Repeatability of the method and or processes. Once we have those elements, the scientific method can be conducted. There are six steps to the scientific method and they are: observation, question, hypothesis, methods, results, and conclusion.
Hydrostatic is relating to or denoting the equilibrium of liquids and the pressure exerted by liquid at rest. So, when the term shock is added, it really becomes an oxymoron in general use. Shock, in the physical realm implies movement, but static implies no movement. So in reality, the "invention" and then gross misuse of this term is likely the greatest cause of confusion. However, as our testing and detection equipment has become more sophisticated, so has our ability to understand and categorize new observations. Put your ear on a railroad track...you can hear the train coming a LONG way before you see it...but even the highest magnification possible of a live SEM is unable to detect any physical change in the metal. Whereas a sound wave in air actually visibly moves molecules.
As for new terms, I would venture to assume that the majority of the people reading this were taught in school that there are three states of matter. It is now accepted in the hard sciences that there are actually 4 states of matter and we have some really smart men applying the scientific method (and they look to be right so far) to prove that there is actually a 5th state of matter.
Are there shock waves of various amplitudes, speeds and frequencies that travel through living tissue upon impact with a moving object? Yes, that can be proven. Does ballistic gelatin replicate those, and the mechanisms? No, that can also be proven. Ballistic gelatin can be used to compare bullets within a certain window and those results can then be projected on to a smaller data set of data from live tissue impact observations and or testing.
There is another very difficult problem when dealing with this subject. It takes medical and engineering (fluid dynamics, materials and ballistics) expertise, and at fairly advanced levels to be able to apply the principles and evaluate the date. So in most cases, it takes a team approach to develop a full understanding of the wound mechanisms and effects on living tissue then the impact that may have on the living organism.
Technology transfer is a term whereby proven methods, systems or operations are taken from one scientific field and applied to another. Since categorizing wounding theories and effects is expensive, time consuming, and for the most part not even possible, technology transfer is a good way to examine some effects. While I have participated in actual live testing, NDAs prevent me from even mentioning any of the elements, people, places, etc., however, the information learned gets to remain in my knowledge base. It is a rather simple matter to approximate impact pressures from bullets. It is then a rather simple matter to find other areas in which similar pressures have been used in other fields. The evaluation and relevance...that is a bit more tricky. Pressure, pressure waves and sonic waves have been used on a variety of meat animals both pre and post mortem in order to evaluate pain, killing efficacy and impact on the internal systems. The goal was processing, health benefits to consumers, texture and taste benefits to consumers and benefits of reduced cooking times. It is clear from the testing done that there are certain levels at which, while there is no visible effect, there was a derived benefit observed. Also, that at various levels, fibrous tearing occurs, cellular rupture occurs and eventually, liquefaction occurs. You have all likely eaten meat that had a shortened cook time, or a flavor alteration due to delivery of pressure and shock waves.
In the medical profession, study of volunteers in crash studies and concussion studies have been able to show that even with no cellular rupture, symptoms do occur in some subjects at certain levels. I am not a doctor, so I can not go into detail, but my general understanding as explained to me is that there is some level between rest and that which ruptures cells (physical damage) in which the "electrical" system of organisms can be short circuited due to pressure events. The affect varies, but can range from a chill, perceived vision flash, tingle to loss of consciousness and in rare cases, cessation of heart and lung function. At least with the other ballistics engineers I deal with in discussing terminal ballistics, it is in this realm, away from that area which suffers physical damage, where this term "hydrostatic shock" has been compartmentalized. None of the ones I have talked to believe that HS is completely reliable nor that is is, in and of itself a killing effect in most cases. When cells are ruptured, one can not term that hydrostatic shock since it is by its very nature dynamic. But the gunwriters, hunters, the majority of lay people and even some purported experts call all of it HS.
Also realize that the majority of the "ballistics experts" are not trained in medicine or engineering...the two fields from which the principles related to terminal ballistics are derived. The engineering and medical terms are often not the same and the crime lab folks borrow from both, but not based on science nor understanding in most cases.
There are multiple types of shock due to impact with an object, and additional types of shock if an object penetrates the flesh of a living organism. Drop in partial pressure of air in the lungs, drop in blood pressure in the circulatory system, psychological, CNS disruption by a variety of methods also occurs.
My education is in engineering (Colorado School of Mines and University of Colorado) with specialization in the thermal-fluid sciences, IC engines and materials. I did some graduate course work and then professional work on cell rupture theory, mostly in the area of microbiology. I have worked for the last 20 years as a forensic engineer. A court endorsed ballistics (as well as some other fields) expert, I have worked for plaintiffs, prosecutors and defendants in civil and criminal trials on a variety of firearms related and other cases including reconstructions. I have consulted for LE and manufacturers related to shooting, weapons, ballistics, design and materials. I am also a lifelong hunter who has shot a lot of game with bullets in testing, some of which did not come to market.
It is my opinion that Marshall and Sanow, started a good work, but with a very limited foundation. I have talked to Evan Marshall on a few occasions, and at one point he sent me a bunch of ammo to test, and I personally believe he tried to make sense of a subject that, at the time was not understandable by one person and lacking many tools now available. I have consumed most of the writings of the Medical professionals, and I believe that Fackler did great work in advancing the general understanding. But I still believe there is more to learn and that the general lay person is still largely making choices and having discussions based on flawed information. Many of the analytical techniques I use to study terminal ballistics were not even available 20 years ago when I graduated college. Even now, the computer time to run some simulations is so expensive, and with all the variables not being nailed down, in most cases it is just not economically feasible. There is still a lot to learn. But, I am not sure most of what will be learned will be of much value to the lay person.
My final thought is this...a handgun is a good compromise between a knife and a rifle...but sometimes you would have only been well served with a tank.