Bob is about to do something that Dave considers stupid. He's going to attempt a Heroic Sacrifice, or otherwise put himself in danger. At this point, Dave could try to reason with Bob, but Dave has a better idea. He punches Bob in the face. (The gut and/or solar plexus are also popular areas.) Nine times out of ten, this will render Bob unconscious, and if it doesn't, it subdues Bob long enough for Dave to take Bob's place or remove him from danger. Dave may get yelled at for this or punched in return if Bob is really put out, but Dave will most likely respond that he did what he had to do.