The gemara says avnu (a rock) and sakin (a knife) are toldos of shor. How can they be like shor? They are very different (one is alive one is not, one moves one doesnt) Tosfos says its like both shor and bor and has charachterisitcs of both. This answer does not work however according to rashi because rashi says that its not like bor. How would rashi explain the sheetah that says a rock and a knife are toldos of shor?
Al, Golus
The Pnei Yehoshua explains that even according to Rashi we can only learn that Avno Sakino u'Masa'oh are Chayav from Shor and Bor together. He says that when Rashi tells us that Avno Sakino u'Masa'oh are not Bor, he means that even though we learn their Chiyuv from Bor, they are not considered Toldos of Bor, since they bear more similarity to Shor.
Dov Freedman