[Jim S]

The only reason for NOT throwing a flag would be if the ball stays inbounds; (assuming B doesn't touch the ball) hits the ground and travels 10 yards untouched by K.
When you come up and determine that there has been a foul, you throw the flag.
Had a play the other nght where after a reception in the EZ the receiver got 'blown up' by a defender. Even from my position at R I could see that it was late, but there was no flag by the covering official.
When I went to him and said I thought that the hit deserved a rag he said he didn't see the hit because he was going up with the TD signal and had already looked away.
That was good enough for me and I threw my flag. If the LJ had time to see the reception, decide to make the call, and start the signal, there was plenty of time for the defender to avoid the hit.
BTW the receiver was on the ground for a good 5 minutes. Enough time for oneof the assistant coaches (who else) to complain loud and long enough to draw the 3/4 USL foul.