I've never heard of this, I thought virtually all bits with a jointed mouth piece and shanks were referred to as tom thumbs and considered not very good bits?
What I'm referring to is that she was told by a "bit expert" apparently that this is a tom thumb and should never be used:
But that is this not a tom thumb and a very good bit:
So what is it? Are they both tom thumbs? Is there any difference in how they act in a horse mouth?