Example #1 (intentional)
You take out a jar of coconut oil and dip your finger in it, you lubricate the insides of the pocket to reduce friction so the ball doesn't rattle in the jaws. You go to shoot and the 9-ball glides in easily.
Example #2 (not intentional)
You eat a cheeseburger and have grease on your fingers, you wipe it off on the cushion a few shots before you get to the 9-ball, you make the shot and the 9-ball glides in easily.