We have the question of why do the chachamim have a k'nas not to let him bring back his hand to reshus ha'yachid.
My question is, why would there be a k'nas in the first place? What did he do wrong? If his hand is still considered part of his body, then he didn't really carry the object anywhere, even if his hand is outside. And if you'll tell since it's outside, it's like a different reshus from his body and that was what he did wrong, then it should be prohibited ma'ikur ha'din to bring his hand back (me'derabonon) since he's bringing from a different reshus to a reshus ha'yachid? And furthermore, if he did it prior to shabbas starting, why would we make a knas if his original action was permitted because it wasn't shabbas yet?
Daniel Altmark, RBS, Israel
The entire Sugya of K'nas is built on the fact that:
a. Placing one's hand in another Reshus does not in itself, render one Chayav Chatas ('Yado Lo Nayach');
b. The fellow did wrong by making an Akirah which will lead to a Chiyuv Chatas, should he complete the act that he began.
As a result of a. there is no basic Isur to withdraw his hand, and as a result of b. the Chachamim either did or did not, issue a K'nas.
This will also explain Tosfos DH 'Mi Kansuhah', who change the word 'Kansuhah' in the first case to 'Asruhah', since, if the Reshus where the hand is has a Din of Karmelis, it will be intrinsically forbidden to withdraw his hand, and does not therefore fall under the category of K'nas.
be'virchas Kol Tuv