My understanding is that a sack is when the QB, on a passing play, is tackled behind the line of scrimmage.
However, in MFN-3, in the Philly/Buffalo game, my QB (Philly) was tackled behind the line of scrimmage 4 times on pass plays, but not one sack was recorded. Even though the QB pulled down the ball and was going to run, it should still be considered a sack. There are many times in the NFL when a QB will get flushed from the pocket, and he will sprint to the sidelines hoping to garner a yard or two, but gets tackled or pushed out of bounds behind the line of scrimmage.. they are always considered sacks.