... >illegal contact occurs when "a player makes significant contact with a receiver after the receiver has advanced five yards beyond the line of scrimmage. This rule was adopted in 1978, and its enactment is regarded as contributing to the dramatic increase in both passing yardage and scoring that the NFL has witnessed since that time."

In 2004, the competition committee made the rule a point of emphasis, essentially penalizing defensive backs for being too aggressive.

If you watched Sunday night's Packers-Bears game, you witnessed another example of this incessantly terrible pen...

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