Crackerjack??

What does the term "crackerjack" mean when discussing bits?

I think you mean "nutcracker". It describes the action of the bit in the horse's mouth. The nutcracker action isn't a good thing, as most horses dislike the feeling. It can pinch the tongue or even hit the roof of the horse's mouth. This happens with single jointed snaffles and tom thumb bits. Any bit with a single joint in the center is capable of having a nutcracker action if used with rough hands. That is why a lot of horses and riders prefer a french link snaffle to a regular one.