A slant route is a pass pattern where the receiver runs at a 45 degree angle toward the middle of the field either from his starting position or after proceeding downfield a few yards.
The point of the route is to take advantage of a defender playing with outside leverage (physically preventing the receiver from breaking outside). It is a quick play and one of the easier throws for a quarterback. The pattern must be quick or inside players, like linebackers, can potentially get into the passing lane to disrupt the play.
It is difficult to defend once the receiver has successfully made his break, so one way of stopping it is to disrupt the timing of the pattern by playing press coverage. Inside leverage (lining up inside the receiver) can be effective in taking the route away as well.
A slant route which is run without first proceeding vertically down the field is sometimes referred to as a quick slant.
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