After a rolling cue ball hits an object ball it's path becomes a curved line, an arc or parabola. The path is not a continuing curved path, but rather an arc followed by a straight path. Once the arc has finished, the forcing part of the shot is finished and the ball travels in a straight line. If you can judge that either the ball or the pocket will stop the ball before the arc is finished, then you have 'forced' the stroke. The more force is used, the more pronounced the arc.
It is very useful to arc the cue ball for a loser when the object ball is near the cushion. By playing with top and pace it is easy to arc the cue ball out into the open and then to a pocket. On some forcing shots top is often required to quickly get the cue ball rolling and to avoid a stun. By arcing the cue ball it sees a much wider pocket. Usually these shots are played into the corner pockets although they can also be played into the middle pockets.
A less apparent shot that utilizes the arc is a rather slow shot when the object ball is close to the cushion. In this case, the cue ball arcs, but before it can come back on line the cushion gets in the way. The cue ball therefore hits the cushion at a much wider angle than the natural half-ball angle.
Skidding and Rolling
The cue ball will initially skid as the tip strikes it. As the ball travels along the cloth friction acts to stop the skid and there is a point when the cue ball will stop skidding and start to roll. The amount of skid depends on the impact point of the tip.
With extreme bottom the cue ball will rotate backwards as it travels forwards but cloth friction will eventually convert this into a skid and eventually a roll.
With extreme top the cue ball will only skid for a fraction of a second as the tip pushes through, and then it will roll. By playing with top you help the ball to start rolling as soon as possible. Furthermore you are less likely to impart any unwanted side onto the cue ball.It is important that players can judge when the ball will be skidding or rolling as it has a massive effect on the shot. E.g. a follow through played with bottom and side will only work if the skid has worn off by the time the cue ball reaches the object ball, and the cue ball is now is rolling nicely.