This is usually a rather intimidating array of different types of bit, each with different features designed to work in a slightly different way.
When the rider takes a contact on the rein, the horse feels a greater amount of contact, depending on the length of the shank.
The bit acts with a nutcracker action (provided it is jointed) on the bars of the mouth (the area of gum between the front and back teeth), the corners of the mouth and the tongue.
As the rider takes a contact on the rein, the horse feels an equal amount of contact on the bit in his mouth.
Another style of snaffle bit is the D-Ring snaffle.
The name is self-explanatory in that the ring of the bit is in the shape of a "D".