The correct answer to this is "bloody painful", having received a couple. The paramedic/ambulance guy rubs - HARD - into a point in the middle of the chest, on the sternum. (Hence sternal rub) It's a pain stimulus designed to tell if you really are unconscious/comatose, or faking it, because it hurts enough that it's kind of difficult to ignore if you're faking it. Oh, and if done truly hard, it leaves a stonking bruise over the sternum for a good week or so.