Sometimes when a player has a ball trapped on a flipper, they really want it to be on the opposite flipper to attempt a particular shot. One way to accomplish this is to flip quickly so that the ball bounces off of the bottom of the slingshot and over to the other flipper.
This technique is one of the safest in pinball and has a very high probability of success. However, it is not 100% foolproof.
Sometimes the ball will just not move and stay trapped. Other times it might bounce too hard and fly off out of control, or it may not bounce far enough and head toward the center drain. As with other techniques, it is up to the player to judge if and when to use this move.
For more, see PAPA.ORG: Intermediate Flipper Techniques