MISHNAH 11a states that if one cuts off ("Ketzitzah") the vine etc. that was Mechubar, the Sukah is Kosher. However, Rav in the Gemara adds that one must also shake ("Ninu'a"- lift up every piece of Sechach and place it down again, see RASHI DH VE'YASIV). SHULCHAN ARUCH OC 626:2 certainly rules like Rav. This means that one needs both Ketzitzah and Ninu'a. Mishneh Berurah 626:13 only writes that Ketzitzah does not have a problem of Ta'aseh ve'Lo Min ha'Asuy, when it is done in conjuction with Ninu'a, which is exactly what the Mechaber says too.

Mishneh Berurah explains why one needs both Ketzitzah and Ninu'a. Because one did Ketzitzah this is considered partially as making a new Sukah and helps remove the problem of Ta'aseh ve'Lo Min ha'Asuy. However, since the vine was trailed on the Sukah and was only afterwards cut off, this is not considered as making a totally new Sukah. Therefore one requires the additional Ninu'a - this is considered equivalent to placing down each piece of Sechach in its new place, and this action completes the total making of the new Sukah.

KESIVAH Ve'CHASIMAH TOVAH

D. Bloom

I have been looking more into Mishnah Berurah 626:13 and there is indeed a difficulty there. Mishnah Berurah writes that even though the Secach was originally connected to the tree, nevertheless this is not considered Ta'aseh ve'Lo Min ha'Asuy since afterwards he cut off the connected Sechach, so this is considered as a new "Asiyah" - "making" of the Sukah, and therefore does not possess the Psul of Ta'aseh.

This statement of Mishnah Berurah would appear to be contradicted by the Sugya because he seems to be saying that "Ketzitzoson Zo He Asiyoson" - "cutting is considered a new making", but this is Rav's opinion, as Rav Huna said, whilst Shmuel disagrees. The Gemara (11b) cites a Beraisa which is difficult for Rav. The Gemara also says there at one point that everyone agrees that one does not say "Ketzitzoson Zo He Asiyoson" and the Sugya certainly seems to be against Rav.

This is in fact how RIF (end of 6a in his pages) rules - against Rav, as does ROSH #21. See MAHARATZ CHIYUS who writes that in order to make the invalid Sechach Kosher one requires 2 actions: 1. to cut it off the tree so that it should no longer be Mechubar, and 2. to shake it so that there should be no problem of "Ta'aseh". This analysis would appear to be consistent with Rif and Rosh, but not with Mishnah Berurah who appears to write that the cutting removes the problem of Ta'aseh.

Shulchan Aruch 626:2 also writes that the shaking removes the problem of Ta'aseh, which is no doubt what you meant when you wrote that Mishnah Berurah is against Shulchan Aruch. See also ME'IRI and RITVA who appear to support Maharatz Chiyus,and see BIRKAS AVRAHAM #10 in our Sugya who points out the difficulty in Mishnah Berurah.

I think however that it may be possible to understand Mishnah Berurah if one looks carefully at RASHI DH VE'YASIV who writes that cutting alone is considered a minor ammendment to the Sukah. It would seem from Rashi, though, that cutting does help a bit to remove "Ta'aseh" but is not enough on its own and requires the additional help of "Ninu'a". Mishnah Berurah explains why "Ketzitzah" is only a minor "Tikun" - because he did not cut it before he trailed the vine on the Sukah. Therefore Mishnah Berurah does not learn like Maharatz Chiyus that Ketzitzah and Ninu'a remove separate problems, but rather learns that together they combine to remove the problem of Ta'aseh.

SHANAH TOVAH U'MESUKAH

D. Bloom