The Broncos aren't the only team that loves to run the WR screen to their playmakers. Out of all the plays I've seen designed to go to Percy Harvin, this is the one that occurs most frequently. The concept is simple. They will almost always run this play out of some sort of trips formation. The way the defense stacks it's receivers provides opportunities for the other two receivers to block down one on one giving the screen man an opportunity at a big play. Due to some inaccurate throws, slips, and good defense, none of these has gone for big yardage thus far.
How to defend it
Like any screen play the players will have to be aware of it's occurrence. If properly blocked, you might have one player in the secondary that can make the play immediately and that requires taking a good initial angle to the ball carrier. This play is down indifferent meaning they could choose to run it at anytime.
Schematically, this is a game where I would specifically account for two people at all times: Russell Wilson, and Percy Harvin. I would assign a player to Wilson...perhaps Von Miller, and I would assign a secondary player to Harvin. Could be the nickel-back Roby or someone else. Even when the team is playing Zone, I would have those two players accounted for at all times. If Wilson is going to get cute with the read option, I would hit him everytime he hands the ball off whether he keeps it or not.
As a RB
This is so out of place, if alarm bells are not going off in the Broncos defenders' heads, something is wrong. If Percy is ever in the backfield in a two-back set where Marshawn Lynch is playing the role of FB, he's getting the ball. And if he's not getting the ball, the Broncos still need to hit him like he's getting the ball. I have no clue what San Diego was thinking here. Everyone including Safety Eric Weddle and the edge rusher crash to the middle of the line of scrimmage leaving the pitch to Harvin wide open to the outside. This is very bad defense and an example of what not to do.
When I coached 8-man football, we ran this exact play to perfection. The wrinkle I added was to time the snap perfectly with the motion so that the man in motion would receive the snap directly. The way Seattle has run it has been from that trips formation coming back to the opposite side of the formation. The thinking being that your trips formation has already pulled at least three defenders to one side of the field. In nickel, which is probably the defensive call against this personnel, that gives you a single high safety and other DB to account for. Essentially you should have a one on one blocking situation at the other end of the formation with potential for a big play.
Notice the read-option component built into this. So far Wilson has given the ball to Harvin, but they notice the Broncos over-aggressively playing Harvin, Wilson can keep and take it the othr way or hand it to Lynch up the middle. This is a multi-layered play with all sorts of possibilities built into it. Since it is likely to come against a nickel formation, there should be a favorable blockers to defenders ratio in the box.
As a receiver
Take note Del Rio, this is on a 3rd and 10. Packers decided to play off-man and they pay for it. You cannot give Percy Harvin this much space off the snap and expect to keep the gain under 10 yards.
Whenever Percy Harvin takes the field, the Broncos defenders must be aware of where he is at all times. If Seattle is lined up in trips formation, they can expect a WR Screen or Jet-Sweep. No matter where he is lined up, Harvin deserves attention on every single snap.
In order to keep is impact to a minimum, the defense must play disciplined--not getting out of place or taking bad angles.
Every time Percy touches the ball, or every time the Broncos think he's going to touch the ball, Percy needs to be hit and hit hard. Let him think twice about tough plays in traffic.
How the Broncos defense fares today depends a great deal on they contain Percy Harvin, and oh Lord have Mercy that's easier said than done.