You can get lost and wander endlessly trying to debate and argue about this subject.
The bottom line often heard is that it seems to have an observable effect in gel blocks, but whether or not it's even going to occur, let alone have an appreciable or consistent effect inside a living threat target/medium ... at the velocities obtained with the common defensive handgun cartridges ... is up for more than a little debate.
Bullets traveling at rifle velocities seem to result in quite a lot of satisfactory anecdotal observations from hunters (with the exception of when they don't).
The muscular-skeletal structure of people, with the various tissues, organs & structures, combined with the angle of presentation and any intervening limbs, would seem to provide a degree of unpredictability when it comes to defensive shootings and placing excessive emphasis on the TC as a wounding/incapacitation factor.
Something else that's sometimes discussed is the 'threshold' velocity seemingly required, and any attendant higher measurable amount of recoil forces produced as a by product of developing that increased velocity, which may affect the shooter and therefore have an adverse influence on the practical accuracy obtained in shooting handguns ... especially if both the attacker/threat and the victim are in erratic motion during the dynamic, chaotic, rapidly changing and evolving encounter.
There are groups of people who fancy themselves as either proponents or detractors of the TC theory. I can't see the need to 'take sides' in this theoretical debate at this time.
Personally, I'd rather spend my time and effort working to develop, improve and refine my individual knowledge, skill sets, techniques, mindset and physical fitness ... using whichever of the common defensive handgun/caliber/ammunition combinations I may have at my disposal at any given time ... instead of becoming involved in theoretical debates.
Misses matter, and not in a good way.
Perforations (also called over-penetrations) matter, and not in a good way.
Peripheral hits on anatomical areas which are shallow and allow perforations aren't often considered helpful or 'immediately effective'.
The psychological reasons for an attacker to stop their threatening actions when shot are not clearly understood or predictable. Factor in some drug/chemical influence and it's even murkier.
Handguns are still just handguns, regardless of an individual's enthusiasm for any particular make/model/caliber or ammunition design.
Even high powered rifle rounds are known to fail to provide an "instant stop", so what does that tell you about relatively (by comparison) low-powered handgun ammunition?
