Why is “yadkha” more “a weak hand” than “there is strength”?

How does the extra heh specifically produce “weak” and not “strong”?

I don’t understand the question. Maybe put tefillin on the leg? The Sages expound “yadkha” as “a weak hand.” It’s a notarikon. How are you inserting “strong hand” here? And why not a leg?

Once and for all,
I need to clarify:
what is a notarikon (not an acronym)?

https://milog.co.il/%D7%A0%D7%95%D7%98%D7%A8%D7%99%D7%A7%D7%95%D7%9F/e_94979

Hello Rabbi, admittedly a bit late, but better late than never.

“And it came to pass, when Pharaoh stubbornly refused to let us go, that the Lord slew all the firstborn in the land of Egypt, from the firstborn of man to the firstborn of beast; therefore I sacrifice to the Lord every male that opens the womb, but all the firstborn of my sons I redeem. And it shall be for a sign upon your hand, and for frontlets between your eyes; for by strength of hand the Lord brought us out of Egypt.”

In the form of the word “yadkha,” there is indeed a hint to place the sign on the weaker hand and not the stronger one, because in the holy tongue, and also in modern Hebrew, words ending with a heh are feminine and softer, like yeled / yaldah, haver / haverah, tamim / temimah, rakh / rakah (“yadkha”). And if you say: but it is the same word referring to the same limb—still, the difference is evident, since the right hand is not at all identical to the other hand in function: cognitive control, writing, “strength of hand,” and so on.
Additionally, from here we also learn that the sign is to be placed on one hand and not both. And conversely, from “by strength of hand” you would learn the reason to place the sign on the strong hand. Here, unlike at the beginning of the passage, the words “by strength of hand” indicate the strength of the hand. By way of illustration, in the expression “with lightheadedness,” the head acted rashly and without thought; and in our case, in “by strength of hand,” in the plain sense the hand acted forcefully (and Heaven forbid to ascribe an actual hand to the Holy One, blessed be He, since “He has no body nor likeness of a body”). And of course, “seemingly with God there is no weak versus strong, but with us there certainly is; and since the essence of the service of God is to resemble His greatness, we use the strong hand to place the sign.”
As for the reasons for the other details in the commandment of tefillin, they are derived from several different places, and the matters are deep and complex.