| The purpose of the West Coast offense is to get the football into the hands of the most talented athletes on the team in a situation that creates favorable mismatches. This is the reason it employs short, quick passes to the receivers and running backs. These plays are low-risk and can deliver the ball to a running back in a one-on-one situation with a linebacker or defensive back, as compared to an ordinary running play, where seven or eight defenders will have an opportunity to tackle the back before he can break free for a long gain.
Signature plays of the West Coast offense include:
The offense will not frequently run the ball outside in the West Coast offense, preferring to run between the tackles and use the screen and swing passes as substitutes for the outside running game. Depending on the quality of the offensive line, passes over the middle may be used to replace the running game entirely. The biggest advantage of the West Coast offense is its flexibility. In order to run it successfully, you need a quarterback who is accurate and has a strong enough arm to complete a quick out, but he does not need to be able to throw deep passes. The offensive line does not need to be particularly talented at run-blocking, and they will rarely be called upon to provide pass protection for more than a couple of seconds. If there are linemen who can move quickly enough to block well on screen plays, that is a bonus. The receivers and backs need good hands, but need not be particularly athletically gifted. If they are, that makes the offense much stronger, of course.
The high quarterback ratings produced by the West Coast offense are a side effect of its emphasis on completing short passes, which generally reduces the risk of interceptions, keeps the completion percentage high, and leads to a passable to good average yards per attempt. The offense works because averaging six yards per completion at an 80% completion rate is fairly simple in this scheme, whereas averaging five yards per rush (which provides a comparable rate of advance--do the math) in the NFL requires a skilled offensive line and a superior running back.
Bill Walsh has written a giant, but truly excellent, book, "Finding the Winning Edge", detailing the offense, the philosophy behind it, and how he implemented it with Stanford and the San Francisco 49ers. If you know the basics of football and want to learn more from a coaching perspective, I highly recommend it. | 
