What information does Defarge get from Jacques on the police force? Where have you heard of this man before?

Jacques recalls the harrowing scene of a man hanging by a chain underneath the Marquis' carriage. This turns out to be an important image for the revolutionaries because it incites enough anger to let them forget about moral sensibilities regarding a possible "war crime." I'm not sure about the last part of your question though but hopefully this helps a little.