Everything. :)

I guess I could just go and look stuff up as it comes to mind. I'm perfectly capable of doing that. I just figured I'd give you guys a chance to show off your knowledge. Makes for good conversation, too.

Do you know the difference between zone and man defense?

No. Nor do I know about screens.

Do we need to pay a premium to learn that?Or are you going to enlighten us for free?

Zone defense is essentially what the Chiefs run when they bust out the Cover 2. The linebackers, safeties and cornerbacks are each responsible for "zones" on the field - certain areas. If any offensive player enters that area, it is their responsibility to cover them.

Before a pass is thrown, the linebackers playing in zone defense will often hit any receiver entering their zones, to disrupt their route and throw off timing. This is legal as it is within the allowed area - five yards and less from the line of scrimmage (where the ball is snapped). The new illegal contact rules make it illegal for defenders down the field to touch receivers.

Man defense, as you might guess, is when each each defender is responsible for another offensive player. A linebacker will run with a tight end, a cornerback with a wide receiver, and so on. You will also see combinations of the two - quite frequently, in fact. In man defense the safeties will often support corners by playing deep zones, to take away long gains - hopefully.

Screen passes are pretty simple. The offensive line allows defensive linemen to penetrate into the offensive backfield. As they release their blocks, they slip out into the flat and block for a running back, who takes a short pass from the quarterback, who is often fading backwards (as if retreating from the pass rush). If it works correctly, it works for a long gain, as the linemen can block defenders downfield and clear space for the running back.

In essence, the screen pass creates a "screen" for the running back to run behind. It's kind of an extended handoff.

Zone = Defending area of the field.
Man = Defending a specific person.
Linebackers can hit WR's to throw them out of line as long as it's witihin 5 yards of the LOS. Otherwise, it's a penalty.
Screens are used to protect the RB in order to try to get a long run.

Here is a slowed-down video of a screen pass.

Note Wiegmann and Waters in the first frame as they release their blocks, before slipping out into the flat to clear space for Priest. 50-yard gain, all because the defensive linemen got too agressive in their pass rush.

http://uranus.ckt.net/~gochiefs/screenpass.gif

Nice video.

You got it. It goes down on the stat sheet as a pass, however.

Also, the way the refs call illegal contact is incredibly inconsistent. I guarantee you Tony Gonzalez gets ridden like a mule farther than five yards down the field.

Ah! I know something else I can learn! Penalties.

Any defensive penalty is 10 yards? Well, I was thinking off sides was 5 yards.

No. Defensive penalties cover 5, 10 and 15 yards and more, if it's pass interference.

Off the top of my head:

Offsides - 5 yards

Encroachment - 5 yards (this is like offsides, but occurs when a defensive player touches another offensive player before the snap. If you go across the line but don't touch anyone, you can try to get back on your own side of the line of scrimmage before they snap and there's no penalty)

Facemask - 5 yards if you just graze the facemask, or 15 yards if you grab the facemask and pull.

Illegal contact - 5 yards, automatic first down

Late hit on the quarterback - 15 yards, automatic first down

Defensive holding - 10 yards

Pass interference - spot foul. If a defender interferes with an offensive player attempting to catch a pass, the offense gets possession of the ball where the interference occured. Even if it's 50 yards down field. In college, the penalty maxes out at 15 yards. They need to change it in the NFL.

I can't recall the name for it, but if you're a defender and you run out of bounds during a play, you can't be the first guy to touch a ball when you come back in bounds. As soon as you touch it, it's a dead ball, and possession is awarded to the team that last had it. This prevents a defender from running out of bounds to avoid a player and then recovering a fumble.

Oh, here's another one:

Hands-to-the-face - 10 yards, I think.

Defensive players can't club a guy in the face with their hands. This is most frequently broke by defensive linemen.

Spearing - 10 yards? You can't lead with your helmet when tackling someone. Risk of injury to both parties.

Off sides and false start are basically the same thing. Offsides is when the defense crosses the LOS before the ball is snapped, and a false start is when the offense does so.

Not exactly. The defense can move around as much as they want before the snap.

Offensive players, once they are set in their stance, are not allowed to move before the snap - unless a receiver, back or tight end goes in motion.