5) The rule clearly states "forward pass." Since doing so after the line of scrimmage would constitute an illegal forward pass, all of the extreme scenarios presented in this thread where the QB runs 20 yards downfield are silly.
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First of all of the two scenarios, one did not suppose the QB crossing the line of scrimmage. Second, when there is an illegal forward pass it is still treated as a pass - if its interecepted the interceptions is allowed. So the illegality of the pass doesn;t make it a non-pass. Couple that with the idea that you are claiming - that the tuck rule makes the focus solely on mechanics, then I don;t see how the officials couldn;t rule it a non-fumble in that extreme scenario: it fits the tuck rule perfectly.