Other designs add straps resembling breeching on the hindquarters to encourage engagement of the horse's rear end.
The use of only a surcingle and side reins, a common component in basic horse training across all equestrian disciplines, is not usually considered a "bitting rig."
It is the use of additional reins such as the overcheck, or the use of leverage to place the horse's head in a set position that turns a classic surcingle into a bitting rig.
Both a bitting rig and a classic surcingle with side reins are intended only to be used while longeing a horse.
While a surcingle and side reins are considered classical training tools, the use of the full bitting rig is controversial in some circles.