This is an excerpt from Pool Player's Edge-2nd Edition by Gerry Kanov & Shari Stauch.
Stop shots, and their stun and drag variations, offer a huge advantage to your game—predictability. These shots are more predictable because you control the cue ball to the greatest degree, with little or no movement after contact with the object ball. As you learned earlier in our discussions of center ball, a straight-in stop shot offers the greatest predictability. The cue ball will come to rest at the exact spot where it contacts the object ball.
When executed on a shot that is not straight in, a stop shot is more often referred to as a stun shot. The cue ball will come off at a predictable 90-degree angle from its contact with the object ball (the tangent line) and will travel as far as the energy left on the cue ball will take it. Nearly full-ball hits with slight angles offer more control; thin cuts will send the cue ball a greater distance, because there's more energy left on the cue ball.
A drag shot is used to send the cue ball a few inches beyond where it contacts the object ball, letting it drift into easy position for your next shot. As opposed to the stop and stun shots, where no forward momentum should be left on the cue ball as it slides into the object ball, a drag shot will leave a bit of forward roll on the cue ball at contact.
Building your skills on these shots, as with all top-shelf shots, requires some focused practice or drills. In each of the drills shown in figure 4.1 through 4.3, attempt to imitate the results shown, and pay strict attention to the reaction of your cue ball on contact with the object ball. Note that as you proceed through the shots in each drill, you will need to increase the speed (force) of the shot or hit farther below center on the cue ball. You'll find that a combination of these two adjustments works well. Next, try achieving each result without increasing the force of your hit and only varying the below-center hit on the cue ball. This might take more practice (and please don't forget to chalk up between every shot!), but the payoff will be more control.
Troubleshooting: If you find that you're not achieving the results predicted by the diagrams, chances are you're making one of these common errors:
1. You may be shooting too hard or too soft. Aim for a medium hit.
2. You may be putting unwanted english on the cue ball.
Check your cue tip's position as it addresses the ball, making sure you're not hitting left or right of the center axis.