"Help me Obi Wan Kenobi - you're our only hope."

The free safety isn't really free. He has to roam waay back and stay there most of the time, just in case somebody (or a group of somebodies) really screw things up in the front there. As you can see, we've got him 20 yards back in this diagram - that's two first downs, out there. And his job is to make Absolutely Positively sure nobody's getting past him.

Because if they do, the team is absolutely screwed, period.

So he roams around back there, watching the game like you and me, with the difference being that where we would love to tackle some jerk who was making a run at our goal line, he actually can. If somebody runs past the linesmen and the linebackers, the free safety pummels him.

Supposedly.

If somebody jets across the field with their arms out, awaiting an incoming ball, he's right there.

Theoretically.

YOU KNOW HE'S DOING HIS JOB WHEN: No matter what happens, by the time the free safety is involved, things mostly suck. If they suck bad, he may have helped keep it from sucking worse.

YOU KNOW HE'S NOT WHEN: Things have gone from totally sucking to totally screwed.