[tom hyle]

The idea of most dropped attacks is that they are done suddenly, taking the opponant by surprise and going under his guard by putting you suddenly below where he thinks you are. There are no magic bullets in hand to hand combat; instantaneity of deadliness is about 90% of it (and I'm leaving asid massed combat, which we aren't discussing and where grouping and communication are probably #1) and is much more important than skill, technique, etc. It's all about speed and anticipation (ESP helps). In other words, this is a trick. The more any such is seen, taught, practiced, and practiced against, the less use it becomes, of course. I remember in SCA heavy weapons combat a trick that only works on a hard smooth floor (or sometimes grass under the right conditions, but not so reliably.....) if you have hard, well-padded knee armour; you charge the opponant head-on, seeming to bull in like an unskilled fool, but at the last instant drop to your knees, sliding in beneath his guard, and strike. It is extremely effective against most people who haven't seen it, because it is unexpected and shifts the attack to a direction from which no attack was thought possible. the technique discussed is rather similar. The idea of the instant kill is a weakness of many sport/training fighting methods. It may be neccessary to them, but it is important for real fighters to train in such things as slide up on your knees then roll away and back to your feet, etc. The move discussed here seems eminently liable to such a recovery that uses gravity/momentum to continue the movement into a roll to the side. Many moves that "kill" an opponant in sport/practice leave you open should he, as we actual animals tend to do, rudely refrain from dying instantly and quietly. I also note a similarity to Spainish bullfighting, though, and to the idea of the perfect thrust, and the perfect understanding and control of the bull, and wherein the armpit of the killer ends up and even comes to rest directly before the point of the weapon of the victim as a very deliberate display that is considered of surpassing meaning and beauty by the afficionadi.
It is sometimes important to remember, too, in these matters, that though such a thing as fear of Death may seem a constant it is actually a cultural construct in some degree, and many cultures have not shared it in anything like the degree it is seen in modern/industrial culture. I recently heard a great saying; I can't remember it precisely, but along the lines of "When we learn to die, then we are able to live free." Killing the enemy, living and dying honorably/morally, and even the aesthetic beauty of movements and situations can all exceed the desire to continue living within traditional combat.
A further note on the soft pass (? low pass?) is that it not only puts you lower than expected, but "ups" your reach, allowing you to strike when the opponant thought he was out of range. There is a very similar extension thrust with a longspear (pike) that can be very effective. The best, most practiced, and stretchiest spearmen can get quite a notable range boost from it.
I'll also add that many fighting styles teach one how to fight from the ground; some even favour it, and styles that lack this typically find it hard to counter, mistaking the ground for a position of weakness; thinking only the feet can successfully interface with it or something; another limitting paradigm whose.....limitations can really come out in a fight.