Here is an A.R.
A.R. 9-1-6-VIII.
Third and seven at the A-30. The ball is at the left hash mark. Back A22 is split completely outside the frame of the tackle on the left side, and B40 moves out to cover him. The handoff goes to back A44 who sweeps around the right end. As the play develops B40 follows the play and A22 chases him. At the A-40 beyond the right hash mark A22 overtakes B40 and blocks him below the waist clearly and directly at the front. The direction of this block is downfield and slightly toward the right sideline. A44 is tackled at the B-45.
RULING: Legal play. The block made directly in front of the opponent is legal no matter which sideline it is toward, since it is not toward Team A’s end line.
If it was illegal to block towards an imaginary line that runs through the ball end line to end line through out the down, then this AR directly contradicts that by saying that the block is legal no matter which sideline it is toward.
Ask 100 players and coaches where the "original position of the ball" was at the snap and they are all going to point back to the general area where the previous play started. If a block occurred 30 yards down field, they surely wouldn't point to a spot in space on that "line".
After you get downfield more than 10 yards and you BBW back towards the "original position of the ball at the snap", you have BBW back towards your own end line anyway.
What I don't think that you can necessarily do is put a time frame on it. I think in most cases that it would have to be rather close to "at the snap" to not be blocking back toward your own end line but in this AR we can see an instance where the blocker does just that and it would not be "at the snap"
A.R. 9-1-6-IV
Back A31, a flanker positioned to the left side of a formation, runs a deep reverse to the right side after receiving the ball from a teammate. End A82 at the snap is positioned 10 yards from the snapper on the right side of the formation. As A31 turns the corner A82 blocks B62 toward the left sideline. The block by A82 is below the waist and generally at the front but outside the “10 o’clock to 2 o’clock” width. The block occurs behind or one yard beyond the neutral zone.
RULING: Illegal block below the waist because it is too much to the opponent’s side and because it is directed toward the original position of the ball. 15-yard penalty. Enforce at the previous spot if the foul occurs behind the neutral zone.
That AR is in the BBW clarification document, but the bolded part above did not make it into the rulebook 2.0. Which is very important in this AR.
In the CFO preseason video in the Illegal Blocks section at about 6:15 into the video there is a block that is fairly close to the snap, I definitely wouldn't say immediate, but the angle that the blocker takes is still back towards the general area where the ball was snapped from and not down the field.
Just my two cents...
