"which is in the saf": Scripture (here) apprises us that he makes an indenture at the side of the threshold and slaughters (the blood) into it. "saf" is a threshold, as in (Ezekiel 43:8) "In setting their threshold ("sippam" [from "saf"]) by My threshold," and (Isaiah 6:4) "And the posts of the sippim would shake." These are the words of R. Yishmael.