The harder the cue ball is struck with english, the more it squirts. When the cue ball is hit lightly, the shaft undergoes very little stress. Hitting harder with english increases the amount of horizontal stress on the shaft. As the cue travels in a straight line, the tip tries to stick to the side of cue ball, which is at an angle from the shaft. This stress causes the shaft to bend in a manner that results in the cue ball squirting from its intended path.
Some shafts have a natural tendency to buckle, while others are very rigid.
A cue deflects because the weight of the cue ball displaces the shaft. If the shaft does not flex, then it will displace the cue ball causing the cue ball to squirt off line. Because deflection is not often noticeable to the eye, many players inadvertently compensate for it without knowing. Some are completely unaware of this and attribute their compensation to either a lack of aiming or a lack of skill. Still, others claim that there is no such thing as deflection.