For any given Clubhead speed at Impact, the maximum ball speed would occur if there were Zero Deceleration of the clubhead between Impact and Seperation of the ball from the Clubface. Of course, that doesn't happen in a real golf swing, but the point is that anything that resists deceleration of the Clubhead during the Impact Interval will increase ball speed. So, the goal is to Sustain the Lag to at least follow through, by which time the ball is long gone.
Place a tee a few inches forward of the ball and try to Sustain the Lag all the way into the tee (a drill shown on Brian's tape).