Originally posted by SnakePlissken:
Let it roll = draw/roll-out audible.
Kill = random audible, but remain at original route/position assignments.
Just to add on to that..
"Let it roll" or another phrase is probably an audible designed by the gameplan. Doubt it will remain the same week after week especially with Harbaugh wanting every advantage he gets.. so he would never telegraph audibles.
"Kill" just mean go to the second option/play called in the huddle. Plays typically have both run and passing options, so its like calling 2 plays from the same formation. Depending on the 2nd call in the huddle will determine whether for example if the kill play was a run up the middle, a toss, or have all WR's run 5 yard comebacks.
Finally an NFL caliber offensive scheme.