Leviticus 1:17

And he shall cut it between its wings, but not to sever the wings from it; and the priest shall burn it at the altar upon the wood which is on the fire: it is a sacrifice, an oblation to be received with favour before the Lord.

He will split as in “completely cleft” [11:3], meaning “cut”

AND HE SHALL REND IT ‘BICHNAFAV’ — “together with its feathers. He need not pluck out the feathers of its plumage. Knafav means the actual feathers [not the wings].” This is the language of Rashi. But it is not correct, for if so, then Scripture does not mention from which place he is to rend [the bird], whether from its front or back. Rather, the meaning thereof is as follows: “he shall rend it at the place of its wings,” for the letter beth [in the word bichnafav] serves here as “the beth of apparatus,” [teaching that the rending is to be done from the back of the bird where “the wings” are, for knafav, as will be explained, means “its wings,” and not “its feathers” as Rashi explained]; similar to the expression, ba’aron, (in the ark), ba’bayith, (in the house), or ba’sadeh (in the field). Knafav does not mean “feathers” [as Rashi wrote], but is similar to all expressions of knafayim mentioned in Scripture [which mean wings]: every bird, whatever hath ‘kanaf’ (wings); and he stretcheth ‘knafav’ (his wings) towards the south. Similarly, ‘bichnaf’ (the skirt) of his robe. Notzah is the down [the soft under-plumage] on the body of birds, something like that which is written, a great eagle with great wings and long pinions, full of ‘hanotzah’ (down). Similarly we have learned [in the Mishnah]: “Large feathers and down are both capable of contracting uncleanness and conveying uncleanness, and join together [with the flesh to make up the required size that is needed to convey uncleanness].” The Rabbis have also said: 12:2. “excepting the beak and claws, the large feathers and the down.”