הכא במאי עסקינן כשיכול להציל (Bava Metzia 22A)

There are 2 different מהלכים in the Rishonim how to explain what the Gemara means here.

1. The way it was explained in shiur which is Rashi's pshat is as follows. The ברייתא is difficult on רבא because the דיוק is that if he is not מתייאש the finder can't keep it which implies that יאוש שלא מדעת לא הוי יאוש. The Gemara answers that רבא has a different דיוק (and consequently a different pshat in the ברייתא). According to רבא the דיוק is that if he can save it (כשיכול להציל) then we don't say יאוש שלא מדעת הוי יאוש because he is not מתייאש. According to this it comes out when the ברייתא says מפני שנתיאשו הבעלים it means יאוש שלא מדעת. The ברייתא said it that way so that we could be מדייק that in a case of יכול להציל there is no יאוש שלא מדעת. As was explained in the shiur the הכא במאי עסקינן is going on the דיוק not the case.

2. Most of the other Rishonim reject Rashi's pshat (as it is very difficult in terms of the language of the Gemara) and explain as follows. הכא במאי עסקינן is changing the אוקימתא of the ברייתא. The answer for רבא is that the case of the ברייתא is a case where he can save it (כשיכול להציל) and therefore there is no דיוק fromמפני שנתיאשו הבעלים that is simply the דין because where he can save it we can't assume that he would be מתייאש. Therefore the ברייתא is not a question on רבא as there is no דיוק, it is simply telling you the דין that in a case where it is יכול להציל there is no יאוש שלא מדעת because we assume that he is not מתייאש.