Here is the rule book's main definition... would need to read the referenced articles for a complete description. That is the (a) part of Rule 12, Section 2, Article 7: A. Players in a defenseless posture are: (1) A player in the act of or just after throwing a pass; (2) A receiver attempting to catch a pass; or who has completed a catch and has not had time to protect himself or has not clearly become a runner. If the receiver/runner is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player; (3) A runner already in the grasp of a tackler and whose forward progress has been stopped; (4) A kickoff or put returner attempting to field a kick in the air; (5) A player on the ground; (6) A kicker/punter during the kick or during the return (Also see Article 6(g) for additional restrictions against a kicker/punter); (7) A quarterback at any time after a change of possession (Also see Article 8(f) for additional restrictions against a quarterback after a change of possession); (8) A player who recieves a "blindside" block when the blocker is moving toward or parallel to his own end line and approaches the opponent from behind or from the side, and (9) A player who is protected from an illegal crackback block (see Article 2). And your all right, even though they have words in there to describe the intent for defenseless... I feel it is left subjective. Was it a fumble? Where does someone have time to protect himself, or becomes a runner? Was he capable? You can debate for weeks.