In the shot diagrammed, we need to pull the cue
ball to the left off the cushion, and the easiest way
is with left English. The first problem when playing a shot like this is that the cueball doesn’t start
out along the line of the stick. Instead it starts out
at an angle away from the side where you apply
the English. This is shown with the exaggerated
arrow to the side labeled “squirt” which is what
this phenomenon is called. Sometimes people
refer to this as “deflection,” but it is actually the
cue stick which deflects while the cue ball squirts.
The second problem is that the cue ball doesn’t travel in a straight line to the object ball, but instead swerves or curves to the left, again shown by the exaggerated curved line. The curve is in the opposite direction, so it might actually cancel the effect of the squirt.
A third problem arises when the cue ball hits the object ball. Instead of driving the object ball straight away from the cueball, the side spin grabs the object ball a little and pulls it to the side.
Each of these aspects of side spin can be more than large enough to make you miss a shot, and each of them has more or less effect depending on the speed, spin and distance of the shot.
Squirt increases as you use more English. If you have a table-length shot, and use a lot of side, you might miss the object ball completely even though you were aiming for a full-ball hit. Squirt also changes for different shaft designs, and both Predator and Meucci Cues have been very active in working to reduce this problem.
Swerve is affected by several other things. It goes up as more spin is used, but stick elevation and the use of draw will also increase the amount of curve you see. Since the curve takes time to develop, if you shoot hard, swerve is reduced. Most players elevate more than they need to, and all players elevate the stick on nearly every shot. Getting the stick flatter is a good way to reduce the guess work when using side spin.
Throw can vary quite a bit with the cleanliness of the balls. If the cue ball is old and covered with ugly green chalk spots, its friction against the object ball will be higher and throw will increase. Throw also seems to be larger for softer hits, but since swerve is also larger then, and they tend to move the object ball in the same direction, it is hard to separate the two effects. If the object ball is close to the pocket, throw is a minor concern, but if the object ball has a long way to go to the pocket, neglecting throw will result in a sure miss. In the same way, squirt and swerve have more effect if the cueball has a long way to go to get to the object ball, and might be safely ignored when the two balls are close.
Some players try to use side spin to put the object ball in the pocket, but this is almost never necessary. There are a few situations in which you need to swerve a little around a blocking ball or to throw the object ball into the needed line, but these are quite rare and for most shots, the added complication of the three problems listed above should warn you away from any needless spin. Keep it simple.
To practice spin shots, set up a shot like the one shown, and see whether you can get the cueball to A, B or C. You will need to set the object ball up in the same place each time because on this shot, the angle of entry to the pocket makes a big difference in how you can manipulate the cue ball. I think you will find that draw combined with left English will get the best results. Start with the cue ball close to the object ball -- within six inches -- and gradually work it back. Try varying the speed and the amount of spin and see how the three factors vary. And remember to chalk!
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