From what I recall, this just means that you're required to do the first 3 steps of your drop back animation before you take control of the QB's movement. You can also throw the ball at any time, you'll just do it off of your back foot and take a huge accuracy/power hit.
A QB's dropback is a very choreographed movement that they do out of muscle memory and almost never deviate from, so this is perfectly realistic. The only thing they might do is throw the ball in a panic if they face an untouched blitzer, and this is accounted for. Really, this is just to prevent people from hiking the ball and immediately running straight sideways to get outside the pocket. So, as far as I see it, +1 realism.