After meeting the Hicks siblings, Sara tells Nick she's happy she's an only child. In a previous episode, Sara told Grissom a story about finding her brother's marijuana and taking it to her mother.
Only someone with no knowledge of liquids, stabbing, biology, or basic physicals could even think that "the first stab is free". For a supposed expert, Finn got everything wrong.
Cast off is blood that is cast off from a weapon during an attack. A knife would have blood on it from the first strike. Therefore, the only free hit with a knife is a total miss.
(The first hit is free in a bludgeoning because there is no blood from the first hit, it just breaks the skin (usually), so the second hit is the first one with cast off.)