A knuckleball is thrown so it doesn’t spin. A baseball isn’t perfectly round and has seams on the surface. These seams affect how the air rushing past the ball flows – creating lift and drag at different random points. The ball gets pushed a little bit in random directions by all the air swirling past, causing it to dance.
I used to watch the NY Mets with my dad when I was a kid – he loved it almost as much as being a chef!
The key to the knuckleball ‘dancing’ is the grip that causes it to spin a lot less than a normal pitch. In fact it only spins 1 and 1/2 times before it gets to the plate versus a normal pitch that spins 8-2o times before it gets to the plate.
Because the knuckle ball spins less and moves much slower (2o mph slower than a regular pitch), other forces such as wind and gravity can act on it more strongly and actually cause it to move in different directions, making it look like it’s dancing and therefore harder to hit (and catch!).