A separate knife is necessary for slaughtering, because the place on the neck at which the Shechitah is performed is considered warm and therefore it causes the knife to absorb blood. The knife, consequently, cannot be used to cut meat that has been kashered (salted, etc.). A separate knife must be used for Chelev, forbidden fat, because although it is not warm, it is prone to get stuck in the teeth of the knife.

D. Zupnik