Tennis balls, so versatile. I almost never play tennis, but they remain useful as mobility tools, for drills like the one above (and others), and behind-the-scenes to protect floors or faces from damage from pointy metal bits. This is one of many drills and games I've found via Ido Portal. Since this video I found better, AKA flat, surfaces to practice it on which made for a more consistent improvement. Compared to traditional (whole hand) wall ball this version is fiendishly difficult to maintain a rally. However that's because you've got to keep many variables consistent on each strike: straight wrist, contact with the ball on particular knuckles (top two for me), measured force on each strike, precise footwork, among others. I like it a lot, recommended for any martial artist and it'd be beneficial for anyone looking to challenge hand-eye and total body coordination.