A flipper consists of hitting the shaft of the diabolo with the stick, changing the trajectory of the diabolo. Usually a grind with a dry hit.
The name of this move comes from the recreational pinball machines, where the flippers are the levers with which you hit the metal ball to prevent it from getting lost in the hole loosing the game. In this case the same thing happens with the diabolo, you use the sticks to hit the metal axis of the diabolo but modify its trajectory.
I like working on these types of tricks because it adds new and fast dynamics to your juggling, they’re not inherently complicated tricks, but they’re not easy to get the hang of for long sequences.