As the cores of the modern-day resin urethane balls became so strong, and as our understanding of the core's influence on the motion of the ball advanced, it became important for bowling ball manufacturers to color the plug so we could see what we were dealing with. Today there is no high-performance bowling ball that doesn't identify where the core is and how far it is from the center of gravity. The center of gravity is marked by a small punch mark on the surface of the bowling ball. The position of the center of gravity relative to the pin location determines how a ball is drilled for the reaction that the bowler is trying to get. Because of today's high tech bowling balls and specially designed weight blocks, the ball driller needs to know where the core is located in the bowling ball.

  Very few high-performance bowling ball labels are located near the pin and the center of gravity. They are all located away from the drilling area, so the ball driller can clearly see the center of gravity and the pin. Only the plastic balls and the lower-priced resin and urethanes still put the label over the center of gravity. Only the plastic and urethane balls with a pancake-style weight block still cover this area. Most of the companies build their cheap balls with no regard to where the pin is, and they color-coordinate the plug to the ball. But they still mark where the center of gravity is by putting the label above it.

  The professional bowler will pick out preferred drillings based on what is felt will work for them. It takes knowledge of the bowling ball's construction. This also requires of how the core and cover stock influence each other and axis coordinates to help decide the drillings.

  When you have bowlers with equal talent that are bowling equally well, the player with the better ball reaction will win. The professional bowler looks at the lane condition and the type of ball reaction they're getting. If they're not getting a good reaction they start thinking about what piece of equipment and drilling configuration will help them get the reaction they need to help maximize their performance.

  What the professional bowlers will look for is the right type of ball movement for the condition they're playing on. If you're bowling right after the lanes are stripped and oiled, you will have a lot of front-end skid and strong back ends, the ball will automatically slide down the lane and finish hard without a lot of effort from the bowling ball or the bowler. On this condition, you need to have the ball set up to have a controlled movement. You don't need a "skid long, flip hard" type of drilling and surface.

  However, late in the day after a lot of bowling, the heads will be hooking early and the back ends will be tight because of lane oil carrydown. The shot will have moved deep, so you'll need a ball that goes long and finishes hard. During any day you may encounter many different ball reactions between those two extremes.

  The bowling ball companies maintain staffs of players on both pro tours, and they also have ball reps out on the tour with the players. The bowling ball rep's will watch the ball reaction as bowlers use his company's balls; when a player requests assistance from the ball rep, he will watch the player to make some recommendations about equipment to use. The ball rep will act as a consultant and sometimes as a coach.

  In order to fully understand what's going on, even the pros who bowl every day need a set of knowledgeable eyes watching their execution and how the ball is reacting to the condition, sometimes it's difficult to determine whether you're throwing the ball properly or not. With the combination of lane conditions and everyone's individual game, it can get very confusing selecting the right ball and proper drilling.      (quoted from: bowling2u.com)