What is a key feature of a breastplate with a running martingale attachment?

A breastplate with a running martingale attachment is designed to keep the saddle in place while allowing for a certain level of movement and comfort for the horse. This is particularly important during activities such as jumping, where the horse's body may shift significantly. The breastplate helps prevent the saddle from slipping back, which could affect the rider's balance and control.

The running martingale part of the breastplate is also crucial because it allows the rider to adjust the reins without overly restricting the horse's head position. This flexible design is beneficial for maintaining communication between horse and rider while still providing the stability that the saddle needs during dynamic movements.

Other options mention aspects like stability during jumping or enhancing speed, but these are not the primary functions of a breastplate with a running martingale attachment. In essence, the key purpose of this equipment is to secure the saddle while ensuring the horse can move freely and comfortably.