Two FBI agents, Carlos and Roger, accept help from a prisoner named Chuck to catch a serial killer. They allow Chuck to meet his old girlfriend Kelly as per his demands. Kelly also knows a little bit of information about the serial killer. Kelly is married to another man.
Carlos: Sir, I was thinking maybe we could use the Kelly woman as insurance in case Chuck gets happy feet.
Roger: Listen, however he plays it, we play it straight. Kelly's involvement remains classified.That means nothing in the report. No name, no location, nothing. Got it?
Carlos: Yes sir.
I know the FBI doesn't usually ruin people's lives to catch serial killers. What does insurance mean here?