At the top of 30b, Rabah states that even though there's a ptur for zakein by hashava, once you start, you're chayiv.

I have heard one reason explained, according to Rashi, is that shitas Rashi is that the aseh of hashava begins once you take the chefetz (see Rashi 26b d.h. "over m'shum hashev te'shevem", Rashi 27a, d.h. "d'asah l'yadei mashma"), and the ptur of zakein by hashava is only for the lav of lo suchal, so since there's no ptur for of zakein for the aseh of hashev, mimayla you become chayiv once you start.

(If this mehalech in Rashi is correct, I have a bit of a question on Rabah later in the same Gemara on 30b telling Abaye that he became chayiv in the mitzvah of hashava after just throwing something at the animal? Aren't you only chayiv after it comes to your hand?)

My main he'ara, is how Rabah's chiddush of a zakein starting the mitzvah of hashava and having to finish relates to the ptur of zakein by prika and ti'ina - would a zakein also have to finish loading/unloading once they start?

The source for the ptur by zakein by ti'ina is not stated outright in the Gemara. Pashtus, like we've been corresponding, the source is the word in the pasuk 'v'hisalamta'. If so, you should also have to finish if you started to load, because the ptur is for the 'lo sir'eh' part of the mitzvah, not for the aseh of 'hakem takim'.

The source for the ptur of zakein by prika is also not stated. Pashtus, it's the word 'v'chidalta', also like we've been saying.

If so, it would come out according to the above mehalech in Rashi, you should be patur from finishing even after you start unloading. The reason is, there's no lav by prika. The pasuk says 'ki sireh', not 'lo sireh' like it does by ti'ina. So if there's only an aseh by prika of 'azov ta'azov', then the ptur for zakein would apply to the entire mitzvah!

Does that follow?

Daniel Steinberg, Columbus, OH USA