The medicine ball is one of Tiny's special moves from ClayFighter and C2: Judgement Clay. The attack involves Tiny Curling himself up into a ball and launching himself at his opponent. This is a charge attack, meaning that you have to hold a directional button back, then press the opposite of that directional button and the button that requires the attack to work (along with the strength of the button). In this case, you have to hold back for at least two seconds, then press forward and punch (any strength will do) to use the attack. This move can also be done in mid-air.
In Judgement clay, however, this move was somewhat different. Instead of being a charge attack, it's a half circle. Just in case you don't know what that is, it's where you take your thumb, put it on the forward or backward directional button, then slide it to the other side while touching the down direction, creating a half circle (hence the title). For this move, you have to press back, than half circle to forward and kick (any strength will do). You can't do it in mid air anymore due to C2's system.
The arcing medicine ball has been changed quite a bit too.