Screw with side
Into a pocket
If you screw back with side in order to get an in-off along a cushion, you need to impart 'non-cushion' side, then the cue ball will spin off the first jaw and into the pocket.
Off a cushion
If you screw back with right side and the cue ball hits a cushion, it will go to the left of you and vice versa. This can be useful in positioning the cue ball for the next shot.