Curbs assist the reins and bit to function crrectly by restricting excess movement of the bit and adding pressure that encourages the pony to react to the bit. When adjusted correctly, the shanks should not exceed a 45 degree angle when the reins are engaged. 
 
The chain curb is better for stronger ponies while the leather curb is milder for ponies who don't need excess pressue. 
 
Stainless steel chain
Comes with two clips for adjusting