I have been lazy and it's been a while since I posted. Here is this past week's shiur.
I spoke about Meleches Machsheves. Rav Menachem Zemba has a chakira in Totzaos Chaim (siman 8) whether we say that the meleches mchsheves makes the action into a melacha or do we say the action was always a melacha but the meleches machsheves creates the chiyuv. Inhis teshuvas Zera Avraham he presents both sides as 2 dinim in meleches machsheves as opposed to a chakira. My understanding of what he is saying in Zera Avraham is that depending on the case, one of the two dinim would apply.
The example he gives is that "zomer v'tzarich l'eitzim-pruning for the wood" is chayav on Shabbos but patur from Shmittah. Tosafos says it is because Shabbos has meleches machsheves. The pshat is that kotzeir b'etzem does not apply to pruning. So for Shemittah we don't view your act as kotzeir. However, since on Shabbos your machshava is to do that act, it becomes a melacha for Shabbos. In this the meleches machsheves creates shem melacha. An example of the other din would be shitas Rabbi Yehuda by melacha sh'ein tzricha l'gufa where you are chayav. You are chayav because according to Rabbi Yehuda, you have enough of a meleches machsheves to create a chiyuv.
I saw that Rav Moshe in his Dibros Moshe in Bava Basra has a similar explanation. The gemara in Baba Kama 61 says if you are zoreh and the wind helps you, you ar patur if you damage someone but chayav for Shabbos. One reason given in the gemara is because of meleches machsheves. Rav Moshe explains that by nezikin we care about your act. So all you did was throw something in the air. The damage was caused by the wind carrying it. However, by Shabbos we care about what you wanted to happen. Your machshava plays a role in deciding if we attribute this action to you. Since you wanted the wind to carry the stalk, you accomplished your goal and we say you did the melacha.
In the Totzoas Chaim he points out that there is a machlokes rishonim how to learn the gemara. The Rosh understands the gemara that since this is how zoreh is done, (and how it was done in the Mishkan-see the Chasam Sofer in Shulchan Aruch Siman 252), therefore you are chayav. The mashmaos is that davka by zoreh we say meleches machsheves makes it a melacha. But other melachos we don't say it. According to the Rosh, meleches machsheves does not give the act a shem melacha but just creates a chiyuv.
Rav Asher Weiss and Totzoas Chaim point out that this is also a machlokes between the Rashba and Ran whether you are chayav for writing on Shabbos for "chok tochos". Chok kTochos is where l'moshol I have a page full of ink and I erase the ink and m'meila I have wriitng, for gittin it is not kesiva. The Rashba holds you are patur onn Shaboos because it is not kesiva and the Ran says meleches machsheves makes it into kesiva. The Ran holds like the tzad that meleches machsheves gives it a shem melacha.
One other nafka mina in the Totzoas Chaim is regarding chatzi shiur on Shabbos. Rashi says chatzi shiur is assur on Shabbos. The Rashbam holds chatzi shiur is patur because it is not meleches machsheves. According to Rashi you can argue that meleches mchsheves does not makes the action into a melacha but rather it just creates the chiyuv. Therefore, even without meleches machsheves I have a shem melacha and can say chatzi shiur is assur.