As the ball is travelling down the left outlane toward the trough, the player hits the front of the machine sharply. If the player’s timing is right, this causes the ball to bounce off of the bottom of the playfield, between the flippers and back into play.
(The following was copypasta’d from PAPA.ORG. View their site for more flipper techniques.)
“The bang back occurs when a player strategically punches the machine cabinet to save a ball draining down the outlane.
“Bang Backs are the king of all illegal pinball techniques. Any players who attempt this maneuver are risking sprained or broken wrists and damage to the pinball machine. This demonstration is for informational purposes only.
“Note: The video refers to a ball draining down the left outlane, but a bang back can be successfully employed from both sides.”
View the rest of the New Pinball Dictionary.