This is a gag snaffle bit, but with a very different effect. A normal gag the gag ropes are attached to the cheek pieces. The curb gag has an extra ring that is attached on top of the bit ring and to the cheek pieces. The gag ropes go through the eyes of the bit, but then they join underneath the chin instead of being attached to the bridle. By pressure on the reins, it will give extra pressure on the lower jaw/chin.
This bit is sharp and give a lot of pressure with a unsteady hand and is therefore not suitable for the inexperienced riders or horses.