Most curbs today have fixed cheeks, meaning that the shanks are molded to the mouthpiece in one solid piece. The slide cheek, though utilized more in the past, can still be found today, and features cheeks that pass through bores on either side of the mouthpiece, much like the rings on a loose-ring snaffle. Typically there is a little bit of play in the cheeks, allowing the mouthpiece to swivel and slide up and down the cheeks by about 6-12 mm (1/4 to 1/2 an inch).
The idea behind this is that when the horse yields to the bit in the poll and jaw, the mouthpiece should slide down the cheeks, thus easing the pressure from the bit. This requires a very finely crafted bit to work properly. Fixed cheek curbs have a more direct action, and are by far more common today.
Interestingly, the slide cheek curb is what was historically meant by the term “Weymouth,” while that term today is used as a catchall for any curb bit used with a double bridle, and in fact is often used to mean the double bridle itself!