On under center:
"Every time you're under center, you've got a lot more run options and a lot more play-action options and a lot more movement options off of your runs," Shanahan said. "Your play choices are endless. You can do everything. Once you get into the 'gun, certain things are like cut in half. Play-action is not as good because it happens quicker. You can't hold the ball out there for as long and stuff. It just eliminates being as balanced."
http://pro32.ap.org/article/it-time-nfl-teams-scale-back-shotgun-offense