As I understand when you are "striking" your joints are extending and also your body mass center is rising.

What about the alternative variation of "dropping the weight" when striking? IMHO, both variations are possible.

What is your opinion about this two "methods" of strinking?

use the Search feature. (left side of your screen).
it's been discussed.

I found one topic where Tim answered: "It depends if you're striking upward or downward.".
There is also one topic with a similar question by "Wind Walker".
However as I understand both variations are possible. Any comment on them?

It depends on the goal and type of strike.

For example, you can "snap" a jab from your elbow very quickly and as long as your body is lined up correctly behind the strike, the blow will potentially have a fair amount of force, but you'll sacrifice maximum power for speed.

If you are mounted on an opponent who is on his back and want to punch him in the face, you will will have to drop your (upper body) weight downward. Would it be possible to strike with "your joints extending and also your body center rising" in this situation?

So, you are correct, there are constants but also variations to generating force in a strike.

Yes, the "theory" of the purpose of the strike seems to be the most logical. :-)
Because both variations are possible. One (rising) seems to be more useful when striking upward and the other (dropping) seems to be more useful when striking downward.