moving your pointer finger and middle finger on the trigger simotaniusly...(i dont think i spelled that right) its alot faster than pulling with both fingers at the same time
its basicaly that when your top finger comes up, your bottom finger is going down, and vice versa, making the gun alot faster and with alot of practice this can become a great skill
What everyone else said, if you want to work on it there is a few sites that can help you with it or just get a pen hold it and jsut start to walk it. It helped me walk faster. I can recomend that you practice with both your hands it will help with when you switch hands in games.