WallBall requires two items. You guessed it: a wall and a ball. Pictured above is a non-reactive medicine ball (or MB). Reactive MBs are those that bounce, while non-reactive (soft) absorb energy. In the photo above, the number "14" refers to pounds.
For WallBall, I have used both reactive and non-reactive MBs of varying weights. Special attention must be given to bouncy reactive MBs during WallBall. I had grown used to the relative little bounce of my soft DynaMax brand MB. The first time I ever tried a reactive MB, it kicked off the wall a bit funny and split my lip open. :P
I like the WallBall exercise for several reasons: It is plyometric and explosive (as most all throwing, catching or "absorbing" drills are). It takes your breath away...fast. It replicates the full-body Thruster exercise; yet WallBall makes extra demands of speed, stamina, and endurance, versus the raw strength and power of Thrusters. It is easy for beginners to learn. It works complex muscle groups simultaneously, building coordination. Finally and in summary, WallBall builds athleticism across so many components of fitness.
The CrossFit standards call for a 10ft tall target and a 20lb ball, but most beginners start with a 10-14lb ball on an 8ft target. "Karen" is the name of the WallBall benchmark exercise. The rules are simple: Do 150 repetitions as quickly as possible. Here is a link to the CF video: WallBall.