The proper strike to the armpit can cause cardiac arrest. (Forget meridians, "chi", kyusho, whathaveyou...) I know this is true.
Response by Dr Ian
Long cardiology lecture: skip this paragraph if not interested.
The heart cycle is: rest while filling, then the top chambers contract (the atria). This event is recorded on an EKG strip as a round bump called the p wave. Then there is another pause and then the large ventricles fire, pumping blood to the lungs and body. This large chunk of muscle causes a large deflection called the QRS complex when it depolarizes, and starts contracting, then another flatline (ST segment) as it contracts, but its electrical state stays depolarized. As it repolarizes, finishing contraction, there's a final T wave deflection.
For most of the cycle if the heart is stimulated it cannot fire again because its busy contracting already. But as the ventricle is repolarizing, just at the end, there's a brief moment where a new stimulus (bump) can start a new contraction (R of the QRS complex) when the heart is not ready athe end of the T, triggering a fluttering wiggle (ventricular fibrillation) that pumps no blood and prevents the normal cycle from resuming.
Out cold in a couple seconds and dead in minutes; only a shock tends to restart the heart; CPR can keep the brain alive a short time while this is obtained.
A student died at uva from this after a lacross stick hit him over his heart, and I read a story in the paper where a student was killed by a karate instructor who kicked him in the left chest.
The thing is, its the tiniest of moments and not every potentially killing jolt gets the job done, or you'd have heard of it before. However, it and the sensation you had (and the GB meridian!) are the reasons we cover our chests in sanchin with the elbows in front and down, punch with the shoulder down, and retract quickly, and hit under the arm at foes in Kanshiwa bunkai.
Left armpit is much more of a risk. And its armpit into the heart rather than up into the arm that does it.