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.
Sweet Iron - will rust when in contact with humidity (air). The surface rust tastes sweet and naturally simulates the salvation production. This will cause the hors to salivate more which will encourage the horse to take more contact.
The process of rusting (oxidisation) changes the blue colour of the Trust bit to a brownish - grey. A sweet iron bit will develop an orange-brow rust layer on the mouthpiece if it is temporarily not used. This is easily removed by wiping it off with a damp cloth.