http://www.ipdb.org/glossary.php#Flipper-Zipper
"Flipper-Zipper —
This is the correct term that Bally gave the feature that is popularly called Zipper Flippers. Their first game to have it is Bally's 1966 'Bazaar'.
It is a trademarked feature found on some of their games that (temporarily) closed (or "zipped" up) the gap between the lowest flippers when certain game goals were accomplished, such as lighting up a series of targets, thereby preventing the ball in play from draining down the center drain. Bally parts catalogs refered to this feature as the Flipper-Zipper Assembly.
Bally used the term on its flyers, starting with their second game to have this feature, Bally's 1966 'Capersville', and they almost always hyphenated it. Bally finally referred to it as 'Zipper Flippers' on the flyer for their last game to have this feature, Bally's 1981 'Medusa', which is also their only Solid State game to have this feature, and their only game where these flippers were not placed at the bottom of the playfield.
Used on four Williams EM games, where it was known as "Closing Flipper Action".
Also used on a few games made in Europe."