Why the lag matters
Most tournament matches start with a lag: both players shoot a ball to the foot rail and back at the same time, and whoever stops closest to the head rail chooses who breaks. Touching a side rail or never reaching the foot rail loses the lag.
More than that, the lag is the purest test of speed control. A ball rolling on cloth slows at a steady rate, so the distance it travels grows with the square of its starting speed: a little more power goes a lot farther. Players who can lag well can control the cue ball for position.
Take it to the table with the perfect lag drill and the practice drills.