An Argentine "Snaffle" (yes, curb) has semi-fixed shanks that swivel. If you look, the headstall attaches in the top ring , the curb strap attaches in the middle ring, and the reins attach at the bottom ring of the shanks. The shanks are angled slightly back, like a traditional curb. This creates a sort-of "door hinge" effect with the shanks. They kinda open and close a little bit against the horse's face, which is supposed to aid the horse with lateral movements. When the reins are slackened on one side, the hinge relaxes and the horse is free to move in that direction without something poking him in the face. A lot of contest bits have this feature. The only problem with it on the AS is that the mouthpiece is broken, which kind of negates the "door hinge" purpose.
The TT (curb) on the other hand does NOT have fixed shanks, and that is what makes this bit so ridiculous. The shanks tip AND swivel. On the TT, the headstall hooks up to the top ring, but then the curb chain goes right underneath it. If you look at it you can see there's 2 rings on the purchase. Then the reins attach at the bottom of the shank. The distance from the mouthpiece to the curb chain is much greater, so there's more curb pressure on the horse's mouth. This distance, coupled with the break in the mouthpiece, can actually make the shanks tip UNDER the horse's jaw - They can swing out, swing forward, swing back, swing under...if you ride it one-handed like you're supposed to, these random actions are greatly reduced, but it's still pretty rough, especially considering all the much better alternatives available to us today. So the mouthpiece in the horse's mouth is doing everything that those shanks are doing, and all of that movement can be amplified by the curb chain.