The game is officially a game of catch played with a
medicine ball under a volleyball net on a tennis court and
with tennis scoring. The official rules are here: http://
www.hooverassociation.org/hooverballrules.html.
There are even regularly scheduled games (http://www.
hooverball.org/) and an annual Hooverball competition
(http://www.hooverassociation.org/hooverfest.html)