Romulan Commander: There's no force that I can use on a Vulcan that will make him speak. That is a fact. But there are Romulan methods completely effective against humans... And human weaknesses.
Spock: You would not resort to them, Commander. They would prove ineffective against the Captain.
Romulan Commander: Then they will leave him dead, or what might be worse than dead.
The Romulan commander is threatening to torture Kirk and possibly kill him in the process.