Guide to Football,” down by contact is addressed through the definition of a tackle. “A player is tackled when one or both of his knees touch the ground,” says the guide. Unfortunately, as is commonplace for beginners guides, the ambiguity of the definition leads to false interpretations.
2012 Official Playing Rules and Casebook of the National Football League discusses the application of down by contact. On page 17, Rule 7, Section 2, Article 1a, a downed ball carrier is described as “when a runner is contacted by a defensive player and touches the ground with any part of his body other than his hands or feet. The ball is dead the instant the runner touches the ground. A runner touching the ground with his hands or feet while in the grasp of an opponent may continue to advance.”
Article 1 of the 2012 Official Playing Rules and Casebook of the National Football League further describes the instances in which players are considered down by contact. Down by contact describes when:
“a runner is held or otherwise restrained so that his forward progress ends” “a quarterback immediately drops to a knee (or simulates dropping to his knee) behind the line of scrimmage” “a runner declares himself down by sliding feet first to the ground”
Thus, down by contact can be summarized as a ball carrier that, as a result of any contact by an opposing player, touches the ground with a part of his body that is not his hands or his feet, or has forward progress halted.