Defensive players love to rush the quarterback, but they have to wait for the quarterback to snap the ball. Sometimes these players get too excited and will cross the line of scrimmage before the ball is snapped. This is called being offside and will cause a five yard penalty to be awarded to the offense.
The offside penalty in football can be committed by the offense and defense and it is when a player moves across the line of scrimmage before the quarterback snaps the football. A quarterback will sometimes use something called a hard count which is a way to try and get the defense to jump into the neutral zone, resulting in an offside penalty. This is a pre-snap penalty so most of the time the play is blown dead before it happens and a five yard penalty will be rewarded.
If the defensive player who jumped offside is not an imposing threat on the quarterback, the play continues and this is called a "free play" because regardless of the result the offense will get at least five yards from the penalty. This can also be committed by the offense, but is a lot less common. Offside is only called on the offense when an offensive player is lined up in the neutral zone. A more common pre-snap penalty for the offense is a false start.
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This is a pre snap penalty that is minor and normally a result of the quarterback drawing a defensive player offside or a defensive player getting antsy behind the line and jumps out of excitement to rush the quarterback.
When a player jumps or is lined up offside in either high school, college, the NFL, or CFL, the referee will award the offensive team five yards and the down will be replayed. This penalty will only result in a first down if there were less than five yards to gain during that play. For example, if it was a 1st and 10, the team would replay the down and have 1st and 5. If it was 2nd and 3 though, the team would be awarded the first down because there were only three yards to gain to get the first down and this penalty awards the team 5. The next play in this instance would result in a 1st and 10.
The AFL is the only league that does not enforce the offside rule because players from both teams have the option to line up on both sides of the ball.
The referee will put both hands on their hips and say something along the lines of: "Offside, defense number 98. This is a 5 yard penalty, repeat 1st down." This is a very basic penalty and is clear cut for the officials to determine.