Deflection
Also known as "squirt", cue ball deflection is the natural result of stroking with sidespin, or "english". When the cue ball is struck to the left or right of center, it does not go exactly in the direction of the stroke of the cue but rather is deflected offline, making it necessary for the player to compensate. The farther the strike is from the center, the more the cue ball is deflected offline. Cue ball deflection also tends to increase somewhat with a faster stroke, for most cues from 2% to 3% over the normal playing range of 5 mph to 15 mph cue ball speed.
Cues vary widely in how much they deflect the cue ball. The lowest deflecting cue that one billiards retailer has tested has only eliminated about 20% of the deflection caused by the average cue. Lower deflecting shafts require less compensation and many believe this equals greater accuracy, but they also tend to spin the ball more and some feel that means less control. Also there is a period of adjustment to a shaft that performs very differently and some are not willing or able to take the time to change from what has become so familiar over many years.
Read more about this topic: Cue Stick