If a punt or KO goes out of bounds through the end zone, it's an automatic possession for the defense, or what we call the receiving team, taken at the 20 ( or the 25, with the new rules). That may be the reason for this rule. I couldn't find any other discussion on the history of this rule, but it's been there forever. The way to avoid it's implementation to to hang onto the rock until you're over the goal line. I'm starting to really get irritated with Gurley's somewhat frequent drops. He has to hang on to that ball. So does Austin, for that matter. Good teams limit their TO's to an absolute minimum, and good teams score TD's in the red zone. Based on that criteria, the Rams have room for improvements.
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