To the OP,
Knockout Power implies more “effectiveness” than merely power
Technique is going to be the most important factor. This means practicing hitting with power, but also with speed, and accuracy so you can actually hit the parts that cause the result. Your “best shot” to my shoulder likely wont yield you as good a result as a “good enough” shot to my chin. Technique work will improve these qualities as well as keeping you from getting put to sleep, since it is difficult to wreck anybody’s shit when you are KO’d yourself.
So, technique first as many folks have written.
Actual weak link training can yield some dividends. However, making sure it translates into hitting harder can be problomatic. A lot of “strength” is joint position and motor pattern specific. So, even if I can out bench you, it may not mean that my shoulder girdle will get more done when we are punching. Schwarzfahrer’s post on weak links is worth re-reading.
If I had to take a SWAG, scientific wild assed guess, about what would yield results witht the majority of people I would look at building up the muscles of the hands and wrists (both grip and hand opening strength, and both wrist flexion and extension). Boxers and kickboxers wrap their hands for a reason. Any buckling in this area when you hit is serious, bad ju ju. The other point would be to strengthen your “core” or “abs”, again because it is the link between the mass of your lower body and the mass of your upper body. The best way to do that is open to some debate.
Here is an old thread that has some decent discussion on the subject. Pay attention to Sento, FightinIrish, and humble in particular.
Regards,
Robert A