As most practitioners realize, the traditional martial arts were developed long before the advent of modern medicine. In those days almost any damage suffered in a fight could ultimately prove fatal via incapacitation, infection, or other collateral impact. Since the shorter duration of the fight the less likelihood of injury to the practitioner, the ancient masters who developed these arts were very concerned about ending fights quickly and decisively. If they could end a confrontation with a single blow so much the better.
Even today, if you lose a fight on the street there is no guarantee that you will walk away without permanent injury, or that you will even walk away at all. If you are lucky you might get to resolve your differences over a beer and a game of pool afterward as I did with the football player at the frat party. If you are unlucky, on the other hand, your adversary may not stop once he or she has beaten you down, continuing to attack until you are in a coma or worse. Even if the bad guy lets you live, you may still be raped, robbed, or violated in various other unpleasant ways before he finishes with you.
If you face off against a skilled attacker such as an experienced street brawler, boxer, or even a fellow martial artist, your opportunities to successfully land a blow during a real fight are limited so you really need to make each one count. The first person who lands a solid blow to a vital area earns a huge advantage even if it doesn't end the fight right away. Defensive movements, techniques commonly thought of as "blocks," can also be fight ending or fight ameliorating if applied properly. At the stadium where I work security I have witnessed several instances where a skilled martial artist broke or dislocated his attacker's arm using a traditional block, ending the confrontation without the need to throw an offensive blow. The defensive movement not only finished the fight but also kept the practitioner out of jail.
Do you have the skill to generate power like that? Short of trying out your martial prowess in a street brawl there are several ways to find out. One method is to tape a couple of thick phone books together and have a partner hold them against his/her chest. Punch the phone books. If you partner feels a pushing sensation or surface pressure you are using external power and/or poor body alignment. Your kime (focus/penetration) is weak. If your partner feels shock deep within his/her chest, on the other hand, you are striking correctly.
A good karate punch delivered in the ikken hissatsu method should rock just about anyone's world, even through two large phone books. Using the whole body to focus internal power rather than "separating" the body in a manner that forces reliance on brute muscle strength is a key aspiration of many martial styles. These punches create instantaneous explosive force, delivering hydrostatic shock deep within the body that disrupts and devastates an adversary. Unfortunately, if you are anything like me, it takes years of diligent practice to get to the point of being able to do that, let alone to do it consistently.
So if ikken hissatsu is so important for real-life street survival, how do you train to develop it? The trick is to work on your kime or focus, delivering techniques with proper body alignment, quickness, and power such that you transmit the full force of your body and all your energy at the moment of impact. The time-honored means of perfecting this ability is through makiwara training.