That bit in your link is a gag. It has shanks on it, but the difference between the shanks on a gag and the shanks on a curb are that the shanks move up and down on a gag. You can see how you attach the reins on the bottom of the longer pieces. When you pull on the reins, they slip and slide through the ring by the mouthpiece.
A regular curb leverage shank should only tip about 45 degrees. That engages the chain and voila! there you go. On a gag, the shanks are actually supposed to tip 90 degree.
Gag bits are designed to pick up the horse's head, and for the rare occasion that proper training has not taught the horse not to run through the bit. Some horses, by profession, need them because running is what they do, so you'll see a lot of TBs in them. It works on the horse's mouth and lips. When you pull on the reins, the shanks slip through and engage the curb chain (which is set very high = increases the severity of leverage). Then the bit makes the same nutcracker motion on the mouth as a regular snaffle (with the addition of the chain squeezing his chin), and the noseband pulls his nose in. The thin mechanical noseband is supposed to hold the horse's head down while the shanks and chain make the horse pick his head up. So that bit makes him do both at the same time. Ouch. =(
That's quite a nasty bit with the noseband and the twisted wire. Gags are nasty to begin with as a general rule (unless you have a specific purpose for using one). I'll go out on a limb and say that people who would use a bit like that probably have no clue what they're doing (not said disrespectfully)** and if they're riding it in only 1 rein, they definitely don't know because you need to ride with a gag rein AND a snaffle rein...most of the time i only see people riding with one rein on a gag. =(
**(I only say that because considering the many advances in training and bits and bitting we've seen, there are MANY more gentle and less drastic bit and exercise alternatives to a bit like that.)
Last edited by Liberty Valance; 07-30-2009 at 08:58 PM.