When a screw-back would not be possible! By raising the butt of your cue and hitting the white at about 45°, slightly below centre, you will get a large amount of screw on the cue ball. Using this method it is quite easy to screw the cue ball back further than the object ball moves - something that would be impossible otherwise. You may wish to reverse your cue grip for this shot.
For more information on this shot, see Piqués.