Deflection (squirt)
"Deflection" can be described as displacement of the cue ball from the aimed direction in the opposite direction of the side english was applied. Like the swerve effect, deflection is an unwanted complicating factor in pool, present whenever english is employed.
The physics of deflection has been studied extensively. Basically, when english is used, the cue ball will always begin its travel in a direction not exactly as aimed; it will "squirt" off of the line parallel with the cue's direction. Deflection increases the faster the cue stick is traveling at impact and the more english has been applied.
Deflection can be decreased by the type of cue used, and at the high levels, players will often select a personal playing cue based on the amount of squirt the cue imparts (the less the better).
Because swerve and deflection (for very different reasons) each cause the cue ball to take a different path than aimed, but each does so in the opposite direction of the other, under the right conditions swerve and deflection can cancel each other out.
Read more about this topic: Cue Sports Techniques