Pretty sure this has been stated somewhere.
Ah here we go, from the Q/A Wiki. Sorry for the formatting, the wiki doesn't lend itself to looking good when copy/pasted
The relevant information is in the second half of the question and answer
Bort
February 11, 2011
Question
Does SYM work on a QB who is running a called pass play, but has pulled the ball down to run? If so, does it depend on if they are outside the pocket as to which ability they get (break tackle v. Spin)? Follow ups on those points: Since it seems that receivers don't turn around and start blocking until the QB has crossed the LOS, I was hoping for a little help in breaking the onslaught of tackles from when the QB tucks the ball and every D player has instant recognition that the QB is running. Also, since all of the defenders have left the receivers open down field, can the scrambling QB untuck the ball and throw it? (Example from 'determined not a bug')
http://goallineblitz.com/game/replay.pl?game_id=1547393&pbp_id=4122961&flash=1 The bug was closed because it was determined that receivers could block, they either 'all' failed a vision check, or (more likely) they cannot commit to block until the QB has crossed the LOS, unless they were already commited to block prior to the pass. Conclusion: Maybe code it so that blockers can react at the same time as defenders, give the QB a chance to beat those defenders with SYM, or let the QB throw it to all those wide open dots.
Bort Answer:
Yes and no to the first two questions. Defenders and blockers both have to pass vision checks to start blocking or following the QB. It may be that the vision checks for the blockers tend to be harder for most blockers (who often have less vision that defenders) to pass vs the high vision defenses out there. The QB cannot currently exit the "run" mode once he's tucked the ball. He can scramble around until he tucks the ball, but I think if he could go back to throw mode, we'd probably see that become pretty overpowered as receivers suddenly were open due to the defenders breaking off. We'd probably have to add more logic and tactics options for likelihood of following the QB when he scrambles, etc.