This is one of the few missable achievements in the game. At the beginning of "The Case of the Chain Reaction" you are at the scene of an accident and have a time limit to try to save as many victims as you can. One of the victims lying on the ground more-or-less in the center, right in front of where you start, needs CPR. Examine him to trigger the QTE, then simply hit the keys in the right pattern. Do well enough and you'll save him and the achievement will unlock.