Jerry Cruncher was a "resurrection" man; in other words, he dug up corpses and sold them, usually to doctors for research. "Recalled to life" was a euphemism for what he did as a living, and it also was a password that allowed Sydney Carton to switch places with Charles Darney for the execution. Sydney would be making the ultimate sacrifice for love and going on to a better life (after life). Jerry Cruncher's occupation would be kept a secret, so he would not end up at the end of a rope (the sentence for grave robbing). Charles Darnay, instead of being executed for being an aristocrat in revolutionary France, would be reunited with his wife and child to live out their natural lives. All in all, it was a very effective signal for letting Carton's plan unfold.