When the cue ball spins, the friction between the ball and the cloth, and the speed at which it travels, causes the cue ball to curve back in the opposite direction from which you hit it. For example, if you strike the cue ball on the right so that it squirts to the left, the spin and the friction on the cloth will actually cause it to move back to the right — a little bit like a boomerang.

The combination of squirt and curve is one of the reasons you would use English for ball-pocketing purposes. Let's say that you are trying to shoot the 4 ball into a pocket and another ball is partially blocking the path between the cue ball and the 4 ball. You might consider applying a little sidespin with a downward stroke of your cue, which would squirt the cue ball out around the blocking object ball and then curve it back toward the 4 ball.