extra 2 seconds to read and react as well as that same extra time for the D-Line to get penetration. Get under the center and smash it in. It also opens up the play-action pass. Fake the hand-off and roll your athletic QB out with a WR shallow in the endzone, another deep (tall WR or TE for a jump ball) and give him the option to tuck and run if it is there. Playing short-yardage out of the gun just seems to be less effective, especially if you are trying to run it in.