Horse People take a look at this bit

[KevT]

Everything I have ever used, the curb strap if needed goes directly under the bit itself. To me it looks like a driving bit with the lines (reins to riders) go on the end, headstall hooks to the other end, and a strap that goes up along the headstall back to the harness goes in the middle. This piece keeps the horse from being able to lower it's head. Less chance of bucking, or kicking and giving you more control over the horse. Mostly used on breast collar harnesses, but have seen it used with collar harnesses too.

 

[Wagtail]

Everything I have ever used, the curb strap if needed goes directly under the bit itself. To me it looks like a driving bit with the lines (reins to riders) go on the end, headstall hooks to the other end, and a strap that goes up along the headstall back to the harness goes in the middle. This piece keeps the horse from being able to lower it's head. Less chance of bucking, or kicking and giving you more control over the horse. Mostly used on breast collar harnesses, but have seen it used with collar harnesses too.

You are correct KevT, the pelham bit is also used as a 'driving bit'.