Safety - Touchback - Forward Progress?

[Bob M.]

REPLY: While Cdnref posted the definition of force and what constitutes a safety, the real issue is whether or not we can rule that forward progress is at work here. If so, the player "carried" a dead ball into the endzone, so 'force' and the idea of a 'safety' would be immaterial. If not, then those definitions would say that the result of the play is a safety. In the original play, there are two questions that need answers:
(1) if the runner's contact with a teammate causes him to fall backward, can forward progress be ruled?
(2) Depending on the answer for the prior question, does it matter if the teammate's contact with the runner is the result of initial contact with an opponent (like dominoes)?