(In reciprocation for KK's help with that Quicksilver and super-speed thread below, I thought I'd post some thoughts on the Karate Kid character.)
Karate Kid traditionally has no 'true' super-power but is supposed to be the peak of normal human potential and the ultimate martial artist. He's been shown to be able to shatter Inertron and down a Daxamite.
How is this possible for a 'normal human'?
One, he definitely has the ability to sense the weak points of his target, perhaps psionically.
Ever since the multi-parter in SB/LoSH (can't recall issue number) where Grimbor has the earth surrounded by energy chains crushing the atmosphere and Val was able to study and find the weak link I have held to this same theory. Despite his claims to powerlessness, I think there must be some psionic ability and perhaps he isn't aware of it
Two, he probably has the ability to focus his efforts down to the molecular level, thus gaining a greater impact effect than a conventional blow would have.
Third, he may be initiating a kind of 'chain reaction' with the ambient energies around him. This means that it's not just his own strength being applied but also the surrounding energies he's directing (i.e. Brownian motion of air molecules and the target, thermal/electromagnetic energies, etc.)
I can see the possibilities in both of these. Given the generic biography that he's studied and incorporated the fighting styles of every culture he's been able to interact with, a blending of styles could make these two theories even more probable than they would be for say, a "regular" human who only had the opportunity to study and develope skills in Terran fighting styles
Fourth, many martial arts stress using the power of your opponent against themselves. Maybe he's psionically affecting the probability of the quantum reaction between himself and his target so that the target takes the damage instead of himself. (There's an incredibly small, though non-zero probability, that a cannon ball will be deflected by a piece of wet tissue paper.)
And you're pretty dead-on here. When he was introduced in Adventure 346 he was pretty cocky and challenged Superboy in order to gain membership. During that fight, a great deal of his end was using Superboy's own strength and momentum against himself. Supes of course changed tactics and wiped the floor with him using his other powers.
This has been shown again even more recently (LoSH post-boot #113) when he uses M'Onel's momentum, redirects it against him and nearly kills him.
Neither of these uses require psionic capability per se, just very fast reflexes and thinking. Though using the opponent's strength against them does not expplain other feats, such as the ability to break inerton, or in Annual 2 of Legionnaires, when he breaks a Daxamite's leg, which is all the more reason I'd buy into your first three ideas as well.
IMHO
Captain Kal
(Been holding this one back too long!)
Don't hold this stuff back -- it isn't every day I get the chance to discuss topics with people who even know jack about Karate Kid
Karate Kid
(Who for the last time did not choose his screen name based on the Ralph Macchio movies)