Just a question that has an answer I am just not sure what it is. Why bag a backward pass? Wouldn't it continue to be part of the run and therefore not a basic spot?

I was speaking of NCAA rules and of backward passes that are thrown when the runner is beyond the NZ. We need that spot as it is the spot that marks the end of a run and we might have a foul that requires we know where the end of run is in order to make thye correct penalty enforcement.

REPLY: That's exactly the reason that you do bag it, because it can be the basic spot. The interval between the time of the backward pass until it is either (1.) possessed again, or (2.) dead by rule is part of the previous running play. Therefore any fouls that occur during this interval, or the run that preceded it, use the spot of the pass (i.e. end of the run) as the basic spot. Consider this play for illustration:
PLAY: 3-10 from midfield. A10 runs to B's 40 where he pitches the ball backwards to A12. During A10's run, B34 grabbed and twisted his facemask at B's 48. A12 catches the pass and runs to B's 35 where he is tackled. RULING: The foul by B34 occurred during A10's run. The basic spot for enforcement of a foul that occurs during a running play is the end of the run. A10's run ended at B's 40 where he threw the backward pass. Enforcement will be from that spot. Result: A, 1-10 from B's 25. If you hadn't bagged the spot of the backward pass, you could not have enforced correctly.

This is exactly the same thing as if A10 fumbled at B's 40.

That is exactly why I asked and in NCAA. I wanted to know what the basic spot is. So, see if this is correct.

A has the ball at the B40. A10 muffs a snap at the B46. While the ball is loose B99 grabs A10's FM and jerks his head. A76 picks up the loose ball and advances to the (a)B30 or (b)B24.

It would be (a)A's ball 1st and 10 from the B25 after accepting the penalty or (b)A's ball 1st and 10 from the B24 after declining the penalty since the foul occurred during the 1st run.

There is only 1 run possible behind the NZ so until the run gets beyond the NZ and ends, we still have 1 run. In your play the enforcement will be from wherever that run ends, 30 or 24. To see the variation I think you are looking for, try this..
Ball snapped at 50. A runs to the B35 and throws a backward pass to a teammate. The teammate advances to the 10 and is tackled there There was a PF facemask by B as the backward pass was being thrownl. That penalty, if accepted would result in a having the ball at the 20. If they decline that penalty, then they get it at the 10.

Slightly off topic, but are illegal forward passes supposed to be blown dead? I've seen plays before where the IFP was only flagged and the play was allowed to continue....

Not blown dead in NCAA rules (unless of course it falls incomplete)

Same under NF rules. Flag it but let the play continue if the pass is completed.

So, under my scenario A gets the ball 1st and 10 from the 15 or 12 if accepted since the penalty is enforced from the end of the run?

In your play, the foul occurred during a muffed snap. This is a loose ball play. Basic spot is the previous spot.

In NCAA rules, yes. There would be no reason to even offer Team A an option of accepting or declining. They are going to get the yardage and the penalty as there is only 1 run on the play and it starts at the snap and ends downfield.

I agree Smiley, but a snap is a backwards pass.

Sorry, I was speaking NF rules.

No problem. Thanks for all the posts from everyone; they helped a lot.